validators: add option for non-ECMA regex validation

[omriarad]

this allows more flexibility with exporting the generated schema with other implementations. according to the JSON schema the pattern SHOULD be ECMA but not necessarily.

Implements #102

[beregond]

Nice, but actually since this is going to be some 'generic' validator - why not create other class for it? I mean ecma=True is just a switch that changes most of bahaviour - it should be distinct validators.GenericRegex - you could simply copy actual Regex and remove flags, translation etc*.

(*Actually since this is generic regex you could/should remove actual validation too - if you don't know what language it is in - you cannot validate it. And if you know (that this is ie PCRE or python expression then such class wouldn't be generic either.)

[avrahamshukron]

In that case maybe validators.PythonRegex class might make sense since we do want to actually validate the value.

[beregond]

Yeah, this would make sense actually, but alongside ECMARegex would be required too. And we cannot remove Regex class anyway for backward compatibility.

[avrahamshukron]

Will work on that soon.

In any case the lack of this validator is not a big deal since the library enables anyone to write such validator themselves

[avrahamshukron]

I think this PR should be closed since it is followed by #108 which fixes the issues with this PR.