Tag attributes, `$context` variable.

Expose tag attributes in the context of the tag directives. Directives
defined within a tag can obtain the attribute values of the attributes
defined on the tag in an `$attibutes` object. Only unqualified
attributes are added to the `$attributes` object.

Additionally, the context is now exposed through the `$context` object,
which may be shortened to `$` in a future commit.

Closes #80.
Strip `xmlns` based directives.

Still no intention of supporting XML, but I am leaving `xmlns`
directives that are not related to Stencil in the documents.

No intention of supporting XML because Stencil is a DOM manipulation
library, a DOM templating library, not a string templating library, and
not an XML templating library. It is indented for web applications,
building documents to be viewed in a web browser. Stencil is not an
HTML5 generator. HTML5 is the chosen standard for serialization of
Stencil documents.

Closes #64.
Merge branch `strip` into `tag`.