APL keymap implemented; slightly cooler installer

[purpleposeidon]

https://github.com/purpleposeidon/xcompose Github is confusing me & it's late.

This uses ♫; as a prefix, which seems fairly pleasant to use, but does conflict with a couple semicolon-related dudes.

[catb0t]

you can create the pull request here: https://github.com/kragen/xcompose/compare/master...purpleposeidon:master (assuming that was your intent)