Add AppVeyor pull request previews [ci skip]

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bc094bd.

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https://travis-ci.com/manubot/rootstock/builds/122622628
https://travis-ci.com/manubot/rootstock/jobs/223977466

The full commit message that triggered this build is copied below:

Add AppVeyor pull request previews

Merges #262
References #198

Add an AppVeyor continuous integration file to enable manuscript build previews in pull requests.
AppVeyorBot comments in pull requests with a link to the latest manuscript PDF.
Move shared continuous integration steps to an install script.
Change the continuous integration skip message in the deploy script to work across multiple continuous integration services.

manuscript.html

@@ -6,7 +6,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
   <meta name="author" content="John Doe" />
   <meta name="author" content="Jane Roe" />
-  <meta name="dcterms.date" content="2019-08-04" />
+  <meta name="dcterms.date" content="2019-08-09" />
   <meta name="keywords" content="markdown, publishing, manubot" />
   <title>Manuscript Title</title>
   <style>
@@ -86,7 +86,7 @@
 <header id="title-block-header">
 <h1 class="title">Manuscript Title</h1>
 </header>
-<p><small><em> This manuscript (<a href="https://manubot.github.io/rootstock/v/579547d8140dd01ddf652dfd4dbeb6c4a2c4b0eb/">permalink</a>) was automatically generated from <a href="https://github.com/manubot/rootstock/tree/579547d8140dd01ddf652dfd4dbeb6c4a2c4b0eb">manubot/rootstock@579547d</a> on August 4, 2019. </em></small></p>
+<p><small><em> This manuscript (<a href="https://manubot.github.io/rootstock/v/bc094bd3d6cec6935298b4acf50e45ad55bf45c8/">permalink</a>) was automatically generated from <a href="https://github.com/manubot/rootstock/tree/bc094bd3d6cec6935298b4acf50e45ad55bf45c8">manubot/rootstock@bc094bd</a> on August 9, 2019. </em></small></p>
 <h2 id="authors">Authors</h2>
 <ul>
 <li><p><strong>John Doe</strong><br> <img src="images/orcid.svg" alt="ORCID icon" class="inline_icon" /> <a href="https://orcid.org/XXXX-XXXX-XXXX-XXXX">XXXX-XXXX-XXXX-XXXX</a> · <img src="images/github.svg" alt="GitHub icon" class="inline_icon" /> <a href="https://github.com/johndoe">johndoe</a> · <img src="images/twitter.svg" alt="Twitter icon" class="inline_icon" /> <a href="https://twitter.com/johndoe">johndoe</a><br> <small> Department of Something, University of Whatever · Funded by Grant XXXXXXXX </small></p></li>

manuscript.md

@@ -2,7 +2,7 @@
 author-meta:
 - John Doe
 - Jane Roe
-date-meta: '2019-08-04'
+date-meta: '2019-08-09'
 keywords:
 - markdown
 - publishing
@@ -18,10 +18,10 @@ title: Manuscript Title
 
 <small><em>
 This manuscript
-([permalink](https://manubot.github.io/rootstock/v/579547d8140dd01ddf652dfd4dbeb6c4a2c4b0eb/))
+([permalink](https://manubot.github.io/rootstock/v/bc094bd3d6cec6935298b4acf50e45ad55bf45c8/))
 was automatically generated
-from [manubot/rootstock@579547d](https://github.com/manubot/rootstock/tree/579547d8140dd01ddf652dfd4dbeb6c4a2c4b0eb)
-on August 4, 2019.
+from [manubot/rootstock@bc094bd](https://github.com/manubot/rootstock/tree/bc094bd3d6cec6935298b4acf50e45ad55bf45c8)
+on August 9, 2019.
 </em></small>
 
 ## Authors

references.json

@@ -438,7 +438,7 @@
         [
           "2019",
           8,
-          4
+          9
         ]
       ]
     },

variables.json

@@ -1,5 +1,5 @@
 {
-  "date": "August 4, 2019",
+  "date": "August 9, 2019",
   "authors": [
     {
       "github": "johndoe",
@@ -46,7 +46,7 @@
     "repo_slug": "manubot/rootstock",
     "repo_owner": "manubot",
     "repo_name": "rootstock",
-    "commit": "579547d8140dd01ddf652dfd4dbeb6c4a2c4b0eb"
+    "commit": "bc094bd3d6cec6935298b4acf50e45ad55bf45c8"
   },
   "manuscript_stats": {
     "reference_counts": {