Add command for case splitting

[txsmith]

This PR accompanies haskell/haskell-ide-engine#530.

Example:

f :: Either Int String -> [Char]
f e = "Hello World"

put your cursor in front of the e identifier and run Haskell: Split case.
The result should be:

f :: Either Int String -> [Char]
f (Left e) = "Hello World"
f (Right e) = "Hello World"

It works as long as the cursor is on the identifier that you wish to generate pattern matches for.
Let me know what you think!

[Tehnix]

Awesome! Let's merge this as soon as haskell/haskell-ide-engine#530 gets merged :)

[alanz]

@txsmith Can you resolve the conflicts, so we can get this merged