Add support for user text objects

[dag]

https://github.com/kana/vim-textobj-user

It would be really cool if we could use the code from :HdevtoolsType to turn Haskell expressions into text objects, such that for example with the cursor on a case we can do cie to delete the whole case expression and enter insert mode, d2ae would delete the case expression and the outer expression containing it (the way repeated :HdevtoolsType works) plus surrounding whitespace, etc.

See also Chris Done's videos on "Structural editing" in emacs using his "syntax-mode":

http://www.youtube.com/user/ChristopherDone