Add lsrc

The `lsrc` command works just like the `rcup` command but instead of
making symlinks and directories, it just lists all the files that would
be symlinks. It prints the destination (e.g. `~/.foo`) and the source
(`~/.dotfiles/foo`), separated by a colon.

Re-write `rcup` in terms of `lsrc`.
Stop ignoring Rakefile and install

Originally my installation script was a Ruby rake task, so I had to
ignore the `Rakefile` when installing.

Then I moved onto `install`, written on a trans-Atlantic flight, so I
had to ignore that too.

Now, ignore neither of these.