Implement str_like()

[hadley]

Which follows the SQL LIKE definition

[quartin]

I have been thinking a little bit about this, and though about the following implementation plan:

• Add a like modifier - str_like default pattern type would be it;
• Have a conversion method from SQL like-clauses to regex and so, under the hood, str_like would use stringr::str_subset

Problems I see here:

• That second point being way harder than what I am imagining, lot's of hammering of edge cases in the conversion;
• Possibility to generate an invalid regex and user sees a confusing error message (but i'm not using regex!).

Is this anything like what you would expect of this feature?

[hadley]

• I was thinking a bit simpler than that — I was imagining just an equivalent of str_detect().

• Yes, I was thinking of using regular expressions to convert like syntax to another regular expression; I think the syntax is simple enough that it's unlikely to cause too many problems.

[rjpat]

Created a PR for this #315 , open to any adjustments