Shake v0.15.6

[Neil Mitchell]

Hi all,

I've just uploaded Shake 0.15.6. A lot of new incremental features
pieces have gone into this release, but should have little visible
impact. Some things that might be of interest:

* You now have ** wildcard patterns as an alternative to //. Hopefully
a future release will encourage ** as a preference. I expect // to
work indefinitely. See
http://neilmitchell.blogspot.co.uk/2016/04/new-shake-with-better-wildcard-patterns.html
for some more info.

* I revamped the website at http://shakebuild.com/ - including a page
on optimising Shake build systems at http://shakebuild.com/profiling.

* I added parallelism operators for user-defined parallelism, see
https://hackage.haskell.org/package/shake-0.15.6/docs/Development-Shake.html#g:7

* There is a forward-defined build system available at
https://hackage.haskell.org/package/shake-0.15.6/docs/Development-Shake-Forward.html
- more for experimental purposes at the moment, but might evolve
further.

The plan is for the next release to be 0.16, and to change the way
custom rule types are defined. The current mechanism is clunky for
users, makes certain types of rule hard to define and some features
hard to implement. I'm still working on the details of what it should
look like.

Thanks, Neil