Include a sass-lint configuration file

[sminnee]

This sass-lint configuration file follows the documented styleguide as
best we can with the current sass-lint rules available, and makes some
notes as to other items that could, in principle, be added as automated
rules.

Note that http://sasstools.github.io/make-sass-lint-config/ was used as
the starting point of this, but manual revision was necessary to get a
good result.

To help developers who are retrospectively applying this styleguide,
the rules that would require non-trivial changes (use of id selectors,
class name formats, nesting levels, and variable names) are flagged as
warnings rather than errors.

[ljharb]

Between this and #23, I think that perhaps we should only add configs if we have a shared way to verify that they match our intentions - like a CSS file they are all run against, or the guide itself.

[sminnee]

Good idea - do you have a reference CSS file available?

[ljharb]

I don't off hand, but we should probably create one that has a 1:1 correspondence with the written guide.

[sminnee]

I'm cleaning up my old pull requests; was there any interest in taking this further?