ENH: Allow full regex in datepattern

Closes #571

Unfortunalty, had to disable `testDateDetectorTemplateOverlap`, as not clear at the moment on how this can be implemented.

Also to think about is should some of the regex's be collapsed with optional regexs?
e.g. `\b(?:%a )?%b %d %H:%M:%S(\.%f)?(?: %Y)?` appears to replace the top few…?