Allow further customization of sass mixins and variables

Currently mixins and variables are (re-)imported into sass partials

```
@import "../variables";
@import "../mixins";
```

https://github.com/google/material-design-lite/blob/master/src/textfield/_textfield.scss#L17

This means mixins, and variables without defaults can't be overridden:

```
@import "material-design-lite/src/variables";
@import "material-design-lite/src/mixins";
@import "project/variables";
@import "project/mixins";
```

The situations where I've run into this could be solved by [using more variables](https://github.com/google/material-design-lite/blob/master/src/_mixins.scss#L213) and [declaring all variables with !default](https://github.com/google/material-design-lite/blob/master/src/_variables.scss#L296) but that still leaves the problem of not being able to re-declare mixins if needed.

```
@import "project/variables";
@import "material-design-lite/src/variables";
@import "material-design-lite/src/mixins";
@import "project/mixins";
```