Feature: Attach baseurl property to template context

[erikjung]

Re: #63, cloudfour/drizzle#43

This change makes the {{baseurl}} template property have a value.

The value will be different depending on the location of each page, and will always be a relative path string pointing to the destination root directory (e.g. dist).

For example:

<!-- src/pages/index.hbs -->
<script src="{{baseurl}}/main.js">

<!-- dist/index.html -->
<script src="./main.js">

<!-- src/pages/sandbox/demos/navigation.hbs -->
<script src="{{baseurl}}/main.js">

<!-- dist/sandbox/demos/navigation.html -->
<script src="../../main.js">

This will allow the static HTML output to be viewed using the file:// protocol without needing any server:

/CC @tylersticka @saralohr @nicolemors @aileenjeffries

[tylersticka]

This sounds a lot more difficult than the code makes it look!

[gerardo-rodriguez]

Any chance we can get documentation? :)

[erikjung]

Sigh. Fine.

[gerardo-rodriguez]

[gerardo-rodriguez]

Looks good @erikjung! Yes, it does sound difficult. :) Do we need to worry about writing any test for this at all?

Otherwise, LGTM! 👍

[erikjung]

@mrgerardorodriguez

Do we need to worry about writing any test for this at all?

I realized that I didn't acknowledge this. Yes, there should eventually.

#93

The tests are going to get a big refactor soon.