Load an initialization file at startup

This file is located either in XDG_CONFIG_HOME or, in case
XDG_CONFIG_HOME is empty, in HOME/.config/gitto.
Separate repository line formatting into format-repository

This way people can override this function in their RC files, and
specify what information they would like to see where.

They would do this with, for example:

,----
| (set! format-repository
|       (lambda (name pushable pullable clean? updated)
|         (format #t "~a: ~d up; ~d down; ~a. Updated ~a\n"
|                 name pushable pullable (if clean? "clean" "dirty")
|                 updated)))
`----

To turn each line into the like of:

,----
| gitto: 1 up; 0 down; dirty. Updated 4 months ago
`----