Add XL breakpoint support

[rdsedmundo]

Description

Closes #232
This PR add support for XL (from Bootstrap 4), and additionally change the all container's width accordingly to it's specs.

Check List

instruction : terminal command

• run the build script grunt
• open index.html in a browser & resize to test visual issues

[hassanasad]

I hope this commit gets pulled in - I have been waiting for this XL breakpoint for months now :)

Thanks @rdsedmundo

[Falieson]

submit it to https://github.com/Falieson/flexboxgrid/pulls

[Falieson]

Read the convo going on at #211 (comment)

[imevro]

Seems like flexboxgrid was abandoned by @kristoferjoseph, so I forked project and made flexboxgrid2.

What is already done:

• added breakpoint xl for 1200px and sm now pointed to 576px (Support for XL like in bootstrap 4 #232) like Bootstrap v4,
• fixed .container class and now it works properly and not causing horizontal scroll (.row causing horizontal scroll when there's content. #254),
• cleaned up from messy structure (In downloaded archive css has different flexboxgrid.css #249)

flexboxgrid2 right now is alpha5, so if you can test it and fills issues (or PRs!) I'll be very grateful.