Suggested: match_fun's as helper functions

[ahcyip]

Suggested feature: Offer the stringdist, interval, etc. match_fun's as helper functions that can be easily input into fuzzy_join.

This would avoid problems like me trying to extract the stringdist function out of stringdist_join :) Here's me trying to use the stringdist match_fun - can you help me figure out what I did wrong here?
Passing arguments into the match_fun function in fuzzyjoin::fuzzy_join

Related to #22 (documentation for match_fun's please!)

[ahcyip]

Oops, actually backticks did not solve the problem. The problem was that the extra arguments for each match_fun (e.g. ignore_case for string_dist) weren't compatible with all of the match_fun's when I wanted to mix and match (use string_dist for one pair of columns, and >= for another pair).