fix(): allow overriding variables

[Manduro]

Short description of what this resolves:

Missing !default on the latest rtl compatible variables introduced by a30379b.

These were the cases I could find, I might have missed some. Maybe it's possible to add a test that checks if all Ionic variables have a !default?

PS: please squash these commits 😬

Ionic Version: 3.3.0

[AmitMY]

Nice work on that! Sorry I missed those.
For future reference - (?!.*?( \!default))^\$.*\; (on all scss files) finds all variables set without !default

@brandyscarney Looks good to me, they didn't miss any variable I changed
What do you think about adding a test, using regex and perhaps an "ignore" rule for the ignored variables like version and paths?

[brandyscarney]

Thanks for the PR! We can discuss a solution for testing @AmitMY. :)