Add an unwanted-branch? function to source-def

To help clean-up obsolete branches, or to delete branches without worrying
about another peer pushing the branch back to you, we'll add an
:unwanted-branches entry to source-def, which is an optional entry that contains
a set of branches to delete automatically. Details are in the
hesokuri.source-def namespace docstring.

This change does not cause the setting to be honored in the source object. It
merely allows it to be read.
Update docstring in hesokuri.source functions

These docstring changes reflect the new live-edit branch feature.
Add feature to hesokuri.repo/delete-branch which allows forcing delete

Forces delete with -D if the optional argument is true. This will be used to
delete unwanted branches specified with :unwanted-branches in source-def.
Repurpose advance-b to also delete unwanted branches

Before, advance-b only delete branches that were peer-originated using the -d
argument. Now it will delete all "unwanted" branches using -D as well.