v6.6.0

Support for other config paths

Added -c, --config to tbump command line, allowing using
a different config file than tbump.toml (based on early work by
@achary)

Support default values for versions fields

Added new [[field]] option for setting default values for version fields
with no match in the version regex (e.g. prerelease fields),
avoiding errors when these fields are present in a version_template.
Patch by @minrk.

For example::

   [[field]]
   name = "prerelease"
   default = ""

Other

• Relax dependency on attrs - we used to have attrs <20, >=19, now we have attrs >= 20.