Add !default suffix to all Sass variables

[vdh]

This change to lessToSass.js adds !default after the end of all variables in Sass files.

One thing that I really like about Sass is variable defaults, so that variables can be defined once at the beginning before importing dependencies instead of having to override variables after the fact. LESS instead has "lazy loading" of variables which makes it technically unneccessary to have defaults, but also causes a lot of confusion about the way LESS vs Sass handle their variables. LESS variables are a lot like Javascript hoisting, which tends to rub me the wrong way. Call me old fashioned, but I just prefer a nice simple top-down variable definition.

For example instead of needing to do this (via sass-loader in Webpack, hence the use of ~):

@import '~react-widgets/lib/scss/variables';
@import '~react-widgets/lib/scss/bootstrap-theme';

@import 'react-widgets-config'; // Custom variable overrides

@import '~react-widgets/lib/scss/mixins';
@import '~react-widgets/lib/scss/normalize';
@import '~react-widgets/lib/scss/icons';

@import '~react-widgets/lib/scss/core';
@import '~react-widgets/lib/scss/popup';
@import '~react-widgets/lib/scss/datepicker';
@import '~react-widgets/lib/scss/selectlist';
@import '~react-widgets/lib/scss/multiselect';

It can be replaced with a much more compact import:

@import 'react-widgets-config';
@import '~react-widgets/lib/scss/react-widgets';

Similar to how someone might do this with LESS and its lazy loading (via less-loader):

@import '~react-widgets/lib/less/react-widgets';
@import 'react-widgets-config';

[vdh]

Rebased to the latest master branch, and removed the changes to lib/* files.