Add typography styles

[HaSistrunk]

Fixes #5 Adds global typography styles including importing Google fonts and adding styles for body, headings, links, and lists. There are currently no media queries to change font size, because preferably we should convert pixels to relative units using a base font size for responsiveness. Instead of doing that in this PR, I think that’s best tackled as a separate issue comprehensively across all styles, and based on our overall approach to spacing and layout (from issue #4 ).

Notes:

• I think component-specific typography should be defined in the styles specific to that component.
• I underlined links by default, meaning we will have to remove underlines for components with links that do not require underlines. I was thinking this might be strategically better to ensure accessibility (to cover WCAG SC 1.4.1 requiring that color is not the only visual means to convey that a link is a link), but that does differ from our approach in the rockarch.org and dimes styles.
• .list--unstyled is its own class instead of being attached to the <ul> styles as it is in dimes, because the linter didn’t like that approach. I’m open to different strategies for that.

Merge #48 first.

[helrond]

@HaSistrunk I'm going to look at this closely in a bit, but would you be able to file an issue for the font size thing with what you know about it now, so that doesn't get lost?

[HaSistrunk]

Nice, this looks good to me. I'll let you merge when you're happy with it.