Add support for disabling apps to build via KERL_CONFIGURE_DISABLE_APPLICATIONS

If non-empty, you can specify the subset of applications to disable when
building (and subsequent installing) Erlang/OTOP with the
KERL_CONFIGURE_DISABLE_APPLICATIONS variable, either in your $HOME/.kerlrc
file or prepending it to the command line.

e.g. KERL_CONFIGURE_DISABLE_APPLICATIONS="odbc"
Merge pull request #54 from norton/dev

Add support for disabling apps to build via KERL_CONFIGURE_DISABLE_APPLICATIONS