Better rake task argument parser that accepts commas in argument values

Current rake task argument parser does not allow arguments to contain commas. This is a pain if one needs to pass such arguments to the rake tasks and users may tend to ugly workarounds such as using a helper character that's later replaced with a comma.

This commit improves the parser so that arguments could contain escaped commas in their values. A backslash is used as the escape character. All escaped characters are passed to the rake tasks as if they were not escaped (i.e. not only commas but any character can be escaped).

Example:
[a\b\\c\,de,fgh] will be interpreted as two arguments "ab\c,de" and "fgh".