Twig 0.0.3

Twig version 0.0.3.

New features:

• Exporting to LaTeX
• Exporting to XML
• Add preference for enabling/disabling spell checking

Fixes:

• Fixed red underline from not showing in misspelled words
• Fixed newlines \\ not creating a new line when using LaTeX math blocks
• Fixed build errors when not using Swift 4.2

Other:

• Use the cmark-gfm-swift Pod, rather than having it as a library