Add $nth parameter for column and span mixins

[filipekiss]

As it was discussed in #405, add an nth parameter to both column and span functions. The parameter may also be set under $jeet.nth if the user wishes to use type by default. The default value is nth-child, so backward compatibility is ensured.

I've updated the documentation and the test, as well the scss results for the playground (I didn't change anything on the playground for stylus since it's already covered by the test).

If you guys need me to change anything before merging, let me know :)

Sorry for the delay, my weekend was kinda rushed and I was only able to clean-up everything this morning.

[corysimmons]

Overall looks good to me, thank you! @pixeldesu Can you fetch this and kick the tires a little then merge?

[corysimmons]

@filipekiss Since this is appended onto the current params, it won't break anything, but it is an additional feature so please bump the middle number here and reset the bug version (so 7.2.0 should be the new version in package.json).

@pixeldesu Once merged, please git pull; npm publish then create a new release:

...with a couple comments describing the release.

[corysimmons]

@filipekiss Thank you! People have been waiting a long time on this.

[corysimmons]

@pixeldesu Can you update website as well? 😬

[filipekiss]

I've updated the package.json file (and quickly greped the project to see if any other place had a reference to the version number, but couldn't find anything). Double-checked if there was something I needed to merge from upstream/master, but everything was up-to-date! I'll take a look at the open issues and see if I can tackle any other of them. :)

[pixeldesu]

Merged, published and released 🎉