Add glyph check API

[dolmen]

The use of Unicode code points (or other glyphs) in plugins can only work properly if the plugin is able to detect if the terminal will be able to display the glyph. We should let the user tell angel-PS1 what font is used to render the terminal and add an API that plugins could use to check if a given glyph is available in the font.

Design idea:

• add a --font command-line option to let the user gives us the path of the font file used to render characters in its terminal
• add a utility function that will parse the font file, extract its glyph names and/or Unicode codepoints. This Adobe tool may help.
• add a utility function that will check if a particular glyph is available

As parsing the font file will probably rely on external tools, this feature should be optional (plugins should be able to work without it).

Useful links:

• https://glyphsapp.com/tutorials/getting-your-glyph-names-right/
• https://github.com/adobe-type-tools/agl-aglfn/blob/master/glyphlist.txt
• https://github.com/adobe-type-tools/afdko/blob/master/FDK/Tools/Programs/tx/source/usage.h

[dolmen]

Another useful tool: xfd (part of Xorg) http://cgit.freedesktop.org/xorg/app/xfd/tree/

[dolmen]

Font::TTF::Cmap should be helpful.

[dolmen]

Modern terminals are Unicode-aware, so I can just close this issue.