add with-cabal, with-compiler to ghcjs-boot

Addresses #154 and #160 - ghcjs-boot now accepts the following new
options:

  --with-cabal - specify an alternative name for the `cabal` program
    to be used while booting. Useful if you used --program-suffix when
    compiling a ghcjs-aware cabal-install.

  --with-compiler - specify an alternative name for the `ghcjs` program.
    I'm not 100% sure why this is helpful, but adding this option should
    fix #154 by ensuring that `ghcjs` is used as the default, regardless
    of the user's cabal configuration.
Fix error during ghcjs-boot

No longer passes the --with-compiler option to cabal when performing a
clean, because that causes it to fail with an unrecognised option
message.

Also added a note in the README.
Merge pull request #161 from hdgarrood/extra-boot-options

add with-cabal, with-compiler to ghcjs-boot