(bug 4650) Git[hub]-enable bootstrap.pl.

Usage: bootstrap.pl --github-user <username> [--dw-nonfree]

Requires the $LJHOME environment variable to be set.
Accepts the user's github username via the --github-user
command-line switch.  If it's not given, falls back to using
the value of the environment variable $GITHUB_USER.

Checks out the user's fork of dw-free to $LJHOME.  Also checks out
dw-nonfree to $LJHOME/ext/dw-nonfree if it is specifically
requested with --dw-nonfree.  Finally, configures Dreamwidth's
corresponding github repositories as upstream for the local
branches 'develop' and 'master'.

Note that git will refuse to clone into a directory that is not
empty (save for dotfiles), so if there are any files in $LJHOME --
like this script, for instance -- cloning dw-free will fail.
Usually one would run this script from the dw user's $HOME directory
with the $LJHOME variable set to something like $HOME/dw.
Discussion here:
https://github.com/dreamwidth/dw-free/pull/355#discussion_r3920545