Divided `mq` into a function and a mixin

Changes :
- Moved core operations from the mixin to a function to allow for _query-chaining_ with the `$and` parameter.
- Added support for the `or` operator.
- Added `mq-stringify()` function, based on `sl-to-string()`, to allow for chaining of `$and` and `$or` parameters.
- Moved computation for `$from` and `$until` boundaries to a function, `mq-parse-breakpoint()`, since its now used 4 times; twice in the function and twice for rasterization.
- Added support for vertical media queries through `$viewport` parameter of `mq()` function and mixin.
- Updated SassDoc and README accordingly.
Updated repo references of 'sass-mq' to 'mcaskill'