Add support for requiring a concept in the query context

This is a quickly implemented solution that only works for a single
concept due to how the validation works.

The behavior is such that no other filters can be added until the required
one is added. From that point on, it can never be removed only changed.

If an invalid state is achieved (initial session), Cilantro will trigger
the 'context:invalid' state and set the `context_invalid` flag on the
session's state.

Handlers can be bound to the event to display a notification when an invalid
state occurs.

Fix #387
Update config.get to pass the default value to getDotProp
Differentiate between preventing an invalid state and asserting one

Only set context_invalid flag on session state when an invalid state is
present.

The other local checks in remove, clear, and enable simply trigger the
event stating the concept is required, but is not declaring the context's
state is invalid. This now triggers a 'context:required' event.
Update return statements when required checks fail
Merge pull request #413 from cbmi/issue-387

Add support for requiring a concept in the query context