Trimming strings, /emacs/lisp/emacs-lisp/subr-x.el modification

[Jean-Christophe Helary]

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

subr-x.el.diff
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