insertCssBefore option to insert esri css before any node using a selector

[tomwayson]

• Describe the proposed changes:

insertCssBefore option lets user insert esri css before any node (like a <style>) using a selector instead of just the nth <link> element. Also loadCss takes this as a second argument instead of overloading the first, which will be an easier API to work with as we tackle issues like #157

This is not a breaking change b/c we haven't released #158 yet

• Is there an example or test page to demonstrate any new or changed features?

no

• Does your PR include appropriate tests for source code alterations?

yes

• If you're adding or changing a public API, did you update the docs Usage sections of the README?

not yet, still thinking about how/when to doc this yes!

• Provide a reference to any related issue.

#107

[jgravois]

lgtm!