Allow work-item association matching to be overridden by user config variable.

Resolve #512 by allowing users to set `git-tfs.work-item-regex` to change the work-item finding expression.
Defaults to the previous behaviour (# followed by a valid work-item id is linked). If the regexp is defined as an empty string then we use the built-in default. If the regexp is invalid then no matching is done.
Added tests to check the above strategy.