Add regexp filter and extend field operator

Implemented the regexp the way we discussed it:

field:FIELDNAME/REGEXP/(MODIFIER)

(in the following examples "[blah]" stands for any allowed parameter. So "/blah/" or "{blah}" could as well be used)

Note that, due to the compatibility with the previous TW5 release, meaning, allowing

    modifier[Jeremy]

to still work, just "by accident" it's still possible to use:

    title:[blah]

to match the title field against the string "blah".

Additionally I allowed the "field" part to be optional, in accordance to the optional "title" operator. So this also works:

    :title[blah]

and is equivalent to

    field:title[blah]

I implemented it this way because otherwise it would be equivalent to

    title:title[blah]

which does not make sense (to me).