Fix conversion of multi-line method arguments

[jonathanhefner]

The regexps responsible for converting method calls with blocks did not account for arguments split across multiple lines. This patch enables multi-line method arguments, as long as every line except the last ends with a comma (and zero or more spaces).

Fixes #40

This patch is intentionally limited in scope. It does not handle lines which end with, e.g. &&. If someone wants to compile a set of use cases, I can make a follow-up PR which tries to accommodate them.