cmds: write .gitignore entries at the end of the file

We were writing new .gitignore entries at the beginning of the file. It
makes more sense to append to this file so that previous .gitignore
entries can be overridden.

Closes #224

Suggested-by: Owen Healy <ellbur@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>