gatsby-remark-prismjs optionally allow additional html entities to be escaped

[patricksimonian]

Summary

Digging into the gatsby-remark-prismjs code highlights that the most common HTML entities ( <, >, ", ```, etc) are escaped using the lodash.escape function. I have personally encountered the requirement to expand on the number of characters escaped to include curly braces (`{`).

Basic example

Allow another option in the plugin to allow for additional characters to be replaced by an encoded version.
Immediately I foresee two ways to accomplish this:

1. The plugin stores a dictionary of all HTML entities and their encoded versions and the plugin option allows the user to pass in a list of entities.

{
   resolve: 'gatsby-remark-prismjs`
   options: {
      ...,
     escapeEntities: [ '}',  '{' ],
    },
}

2. The plugin allows a dictionary to be passed in so that the user is in control of how values get replaced.

{
   resolve: 'gatsby-remark-prismjs`
   options: {
      ...,
     escapeEntities: {
          '{': '{',
      }
    },
}

Motivation

During the support of my application, I encountered a very hard to debug the issue. The static site Server I was using (Caddy Server) was evaluating ansible handle bar code examples {{ .. }} as valid Caddy template variables. This was causing my server to crash.

To me, the most sensible solution allows for additional HTML entities to be escaped.

I initially authored my own remark plugin to escape these entities...which did work but unfortunately, it is not compatible with the remark-prismjs because it requires changing the mdast node.type

[patricksimonian]

I'm also happy to make a PR for these changes if the approach is acceptable

[LekoArts]

Option 2 seems like a good idea. Would appreciate a PR!