Add get-value method to Context protocol.
Implement new function of Context protocol for continuation monad.
Implement new function of Context protocol for either monad.
Implement new function of Context protocol for identity monad.
Implement new function of Context protocol for maybe monad.
Implement new function of Context protocol for state monad.
Implement new function of Context protocol for seq, vector and set monad.
Add some tests for get-value function with maybe monad.
Merge pull request #12 from niwibe/uniform-extract-value

Uniform extract value from monad context.