Add git-dch --customizations FILE to allow changelog entry customization

Add git-dch --customizations FILE to allow changelog entry customization

Add support for git-dch --customizations FILE. FILE must be Python
code, and for now, the only useful thing it can do is define a
format_changelog_entry() function which will override
gbp.dch.format_changelog_entry().

Add a new customization option group for --customizations.

Create a gbp.dch module and move the changelog entry formatting
functions there.  Create separate procedures to handle extracting
metadata from the git log, and use them in the default
format_changelog_entry().  These functions are also available for use
by custom formatters: extract_git_dch_cmds(),
filter_ignore_rx_matches(), extract_bts_cmds(), extract_thanks_info(),
etc.

Add a GitRepository.get_commit_info() method, and use it in git-dch
parse_commit().

Signed-off-by: Rob Browning <rlb@defaultvalue.org>