new setup script

[aryarm]

Note: Make sure you decide whether to merge this before #87.

I wrote this PR primarily to resolve #72, but then things sort of snowballed, and I ended up refactoring a lot of setup. Feel free to keep or change code as you please!

Here are the major changes:

• setup will prepend to .bash_aliases instead of appending to it (see #72 for discussion)
• remove setup's realpath dependency See this discussion for why I don't think we need it.
• setup will update $ABSPATH if the user decides to change the location of the myaliases directory The user should be encouraged to run setup every time they move myaliases.
• support for spaces in $ABSPATH (although admittedly this isn't tested)
• improve portability
  • for example, I use $HOME instead of ~
• To resolve #80, I deleted the code that would source ~/.bashrc It's impossible to source a file from inside of a script in the way you wanted to, since scripts are executed in subshells. Instead, we can just tell the user to do it themselves, or they can use the src command if they feel more comfortable with that. In a way, it's almost better that they handle it themselves because we use $SHELL to detect their shell, but $SHELL refers to their default shell, not necessarily the one they're currently using. By the way, I updated the src command to improve portability (since some environments might have their default shell installed locally instead of in /bin).

I put portability at the forefront of these changes. The setup/uninstall scripts should now work with any POSIX compatible shells. This is relevant for #28.

This version of setup is incompatible with the original. Before trying out the new setup, the user should first run the old uninstall script.

[GiselleSerate]

Oh boy, this merged into prepend. Did not realize. Oops. Got it.