Implemented option for comma separated slice data

[Southclaws]

Resolves #2

• Encoding struct fields with comma option set results in a single parameter with comma separated values. It's the most basic implementation and will not attempt to escape commas that exist within strings.
• Decoding struct fields with comma option set will treat input as a single url.Values item and run strings.Split on it to produce an iterable for conversion. Will not attempt to handle escaped commas within strings. Only tested with primitive types.
• Documented changes underneath the omitempty note.

[mariusor]

Probably you meant to reference the issue #2, not PR #1.

Also it seems that the project is somewhat abandoned. I didn't get as much as a ping from the owners for my PR opened more than a month ago.

[Southclaws]

Yeah I forgot to check the repo activity!

In that case, feel free to PR your addition to my fork, might as well keep a single copy for future users.