Enable using a RegExp as delimiter

This fixes a regression that was introduced in #16 which prevents
anything but a string from being used as a delimiter. But the whole
point of #12 was to allow users to specify a RegExp to use in
queryString.split(delimiter).
Merge pull request #22 from mjackson/regexp-delim

Enable using a RegExp as delimiter
update docs
2.1.0
allow Infinity for parameterLimit, closes #23