Add 'declaration' option to control generation of XML declaration in output

[srowen]

This adds a new option, declaration, which controls the contents of an XML declaration at the very start of each output XML document, like <?xml ... ?>. Defaults to generating <?xml version="1.0" encoding="UTF-8" standalone="yes"?> but can be disabled by setting the option to null or empty string.

[codecov-commenter]

Codecov Report

Merging #560 (2754da8) into master (80f8737) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   85.49%   85.50%   +0.01%     
==========================================
  Files          18       18              
  Lines         965      973       +8     
  Branches       92       85       -7     
==========================================
+ Hits          825      832       +7     
- Misses        140      141       +1     

Impacted Files	Coverage Δ
...in/scala/com/databricks/spark/xml/XmlOptions.scala	97.61% <75.00%> (-2.39%)	⬇️
.../scala/com/databricks/spark/xml/util/XmlFile.scala	100.00% <100.00%> (ø)

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