Set up unit tests

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Unit tests for the components are both very important, and also a thing I have very little idea of how I'll go about it.

The main issue being that this crate depends on some npm packages (specifically, the @material/mdc-* component classes). I'll have to figure out how to go about including such external dependencies in unit tests.

• Set up basic wasm testing
• Figure out how to include JS dependencies
• Set up Travis CI

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Tests can be run with wasm-pack test --<browser> --headless, where <browser> is one of firefox, chrome and safari. (See: https://rustwasm.github.io/docs/wasm-bindgen/wasm-bindgen-test/browsers.html)
There's no point in running tests in node because this is a browser-only crate.

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Research, as of now, resulted in:

• wasm-bindgen-test-runner is using a relatively simple internal implementation of the WebDriver protocol to execute the browser tests.
• Their approach works fine for anything that only required Rust dependencies.

I think that in any case, tests with JS dependencies will have to be their own projects - because they will also have to be Node projects of some kind.
I am not yet sure how to integrate that with wasm-bindgen-test and the general test facilities; if it's possible to run a project as a test, that would be the way to go, because then all I'd need to do would be running npm install from a build script.

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Update: I just realized that while unit tests are hard to write here for little benefit, doctests are absolutely worth it; since yew component initialization is fully compile-time checked, writing one doc test per MDC component would at least ensure that it can be initialized as intended, and that would also help with #2 a lot (as the library is still really lacking there).