caveats first-class in DSL, gets own mini-DSL

Make caveats a first-class member of the Cask DSL.  It no longer has to
be specified with "def caveats", but can be given as "caveats do ... end"
as with after_install blocks.

In addition, create a mini-DSL which can be used within caveats blocks,
providing standard messages when manual installers must be used,
reboot-required, etc.

Add alternate form: caveats can also accept a compile-time string for
simple cases.

This is 99% compatible with old Casks, as it still works by defining
a caveats method.  However, all Casks containing caveats are cleaned
up and adapted according to the new DSL.

Full docs in CONTRIBUTING.md.