Simplified expression syntax.

It truly looks like RegExp nightmare and manages to transform this:

'main field from my document' . sub_field == "some nasty string we've got here gov'na!!"

into this:

doc["main field from my document"]["sub_field"] == "some nasty string we've got
here gov'na!!"

I.e. "string literals" are preserved (even with single quotation marks) and
'variable names' are interpreted as doc members. Unfortunately the \w pattern
does not include äöß etc. so these had to be added explicitely. Also, I should
add a couple of comments because this one RegExp is overwhelmingly long.
Rephrased _expression option label.

Since it isn't JavaScript anymore, I exchanged the label with something I also
don't like. It reminds me of Excel.