add possibility of persistent configuration

options are stored in as git options in the group sanity.<dirname>, with
<dirname> being the basename of the repository.
it is recommended that these keys are added to the global git config,
outside the clones of the inspected repos.

the only currently recognized option is "flags", which corresponds to
the GIT_PUSH environment variable.

Change-Id: I011d4f335ca94b5aa08fc9bcb6283e6c2f7b3b79
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>