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.
James Moon