Improve fontification of sub- and superscript

[saf-dmitry]

Improve fontification of sub- and superscript expressions

Description

Limiting sub- and superscript expressions to only alphanumerical characters is too restrictive. Now sub- and superscript expressions can contain backslash-escaped spaces, punctuation marks, and other symbols, accrording to Pandoc syntax: http://pandoc.org/MANUAL.html#superscripts-and-subscripts.

Related Issue

This commit should close #346.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).