Add highlight_options configuration value

This changeset adds the `highlight_options` configuration value, which
specifies a dict of key-value pairs to pass to the Pygments highlighter
specified with `highlight_language`. For example, specifying
`highlight_options = {'funcnamehighlighting' : False}` for the 'php'
lexer will disable function name highlighting.
Both the HTML and LaTeX writers have been updated to include the
options when calling `highlight_block()`, as long as the currently
selected language matches what was set with `highlight_language`, i.e.
not when modified by `highlight::` or `code-block::` directives.

CHANGES

@@ -9,6 +9,8 @@ Incompatible changes
 Features added
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+* Added ``highlight_options`` configuration value.
+
 Bugs fixed
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