`WithFilter` lets you supply an event filter that will be invoked
before Bubble Tea processes a `tea.Msg`. The event filter can return
any `tea.Msg` which will then get handled by Bubble Tea instead of
the original event. If the event filter returns nil, the event
will be ignored and Bubble Tea will not process it.
As an example, this could be used to prevent a program from
shutting down if there are unsaved changes.
Based on the fantastic work by @aschey and supersedes #521.
Resolves#472.
WithContext lets you specify a context in which to run the Program.
This is useful if you want to cancel the execution from outside.
When a Program gets cancelled it will exit with an error
ErrProgramKilled.
This commit also contains some refactors:
* Refactor away inputStatus type
* Refactor away program.inputIsTTY member
* Simplify how we setup and restore input when it's a TTY
