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.
better user.test
WIP
Make >=> and <=< context-aware
outer method in monad transformers in order to achieve automatic lifting
MonadState protocol
Implement MonadState in the state monad
`ask` reader for getting access to the reader environment
Make ask a reader and actually test it
`local` function for modifying reader environments
Reader monad transformer
A maybe monad transformer wraps a maybe in the given monad, not the other way around