Support the PCRE Regex PARTIAL_{HARD,SOFT} opts.

[NHDaly]

Make exec aware of the PARTIAL_HARD and PARTIAL_SOFT flags set on a Regex object.

This allows match() to return partial matches if they are requested by the user in the Regex options. Currently we are ignoring these options, since partial matches are returned as a -2 status code, and we are not currently checking for that.

---

Partial Regex matches are often used for interactive validation of UI fields (e.g. in a web page):

• You could have a regex describing the format of the field you expect, which you check for a (partial) match as the user types, so you can catch as soon as they have entered something that is invalid (because no completion of it will satisfy the regex), and underline the text in red (or something).
• For example, something like this:

  if (partial_match(r"\d\d/\d\d/\d\d\d\d", input) == nothing) throw("input must be a \"dd/mm/yyyy\" string") end

---

I didn't include a convenience function for constructing a partial regex, which maybe we should also include in this PR?
Right now we're constructing them in our code like this:

partial(str::AbstractString) = Regex(str, Base.DEFAULT_COMPILER_OPTS,
    Base.DEFAULT_MATCH_OPTS | Base.PCRE.PARTIAL_HARD)

But it might make sense to add a flag to the r"" macro, in the same style as the m/i flags? I couldn't find any equivalent such flags online -- where is the inspiration for those one-letter flags taken from?

Or this could be done in follow-up PRs.

[rfourquet]

Ouch I'm sorry, I just opened a competing PR, I didn't think of checking earlier on github before starting mine! Quite funny that we thought about doing the same thing within a 3-days interval :)