[WIP] New feature: add AGF sources

[alfredclwong]

Fixes #107

Main commit is e9c1936.

Defines GlassCat.add_agf(sourcefile::AbstractString; name::Maybe{AbstractString} = nothing, rebuild::Bool = true), which allows the user to add custom AGF files to sources.txt at runtime. These will then be incorporated into AGFGlassCat.jl on the next rebuild.

TODO:

• Write tests for old (and new) GlassCat build functions
• Update docs to reflect changes [PREVIEW]
• Receive feedback

[BrianGun]

Will we ever want catalog names that are not purely alphabetical? I can extend the regex to match alphanumeric names with a non-numeric leading character.

for now alphabetical is fine.