obsolete: add isenabled function for option checking
Previously, obsolete used the module level _enabled flag to determine whether it
was on or off. We need a bit more granular control, so we'll be introducing
toggle options. The isenabled() function is how you check if a particular option
is enabled for the given repository.
Future patches will add options such as 'createmarkers', 'allowunstable', and
'exchange' to enable various features of obsolete markers.