Add support for NerdFont glyphs

[tonesandtones]

Added support for NerdFont glyphs. Implements issue #20

If the codepoint is known to be used by NerdFonts, then replace the standard message with one specific to NerdFonts:

• Links to the NerdFonts cheat sheet page for the codepoint. For example, https://www.nerdfonts.com/cheat-sheet?set=f2a2 (nf-fa-assistive_listening_systems)
• Uses a markdown code block to render the glyph. Markdown code blocks use the editor font instead of the system font. If you're working with NerdFonts codepoints then your editor font is likely to be a NerdFonts font and thus contains glyphs for the NF codepoints, whereas the system font is almost certainly not a NerdFonts font.