Following the discussion about how trimming in subr-x.el did not allow for non-default regex, I have this small modification to the 3 trimming functions found therein.

Basically:
1) I extracted the default regex and assigned it to string-trim-default-regex
2) I declared optional "trim-left" and "trim-right" arguments respectively for the string-trim-left and string-trim-right functions, and declared both optional for the general string-trim function

The documentation strings are pretty much taken from subr.el so maybe the wording is not the best.

Please advise.

Jean-Christophe