Allow run-cabal-test to specify cabal executable name

[killy971]

Introduce an optional argument to the run-cabal-test command which will allow to specify a custom command name to use instead of cabal. The argument will be named --cabal-name, of default value cabal.

This will simplify Travis setups such as the ones based on multi-ghc-travis, for example to replace:

  - ln -s /usr/bin/cabal-1.18 ~/.cabal/bin/cabal
  - run-cabal-test

by:

  - run-cabal-test --cabal-name=cabal-1.18

[killy971]

• implemented in 557390f684b411f41a62d8f5394e45116c537a3a, 107d64e02f54b048603e7a903157a3d784369c94 and f6859c1b92216767d257752fd9a8cea065928bba
• merged into master in 942916d31423880075b46b9cf44c8acec64ac89c
• to be released in version 0.2.0

Example to use cabal-1.18 instead of cabal:

run-cabal-test --cabal-name=cabal-1.18

Original cabal test options can still be used, as in the following example:

run-cabal-test --cabal-name=cabal-1.18 --show-details=always

The current implementation doesn't rely on the cmdargs package at the moment, for simplification.