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
Update README.md

* Add a note on Tuning section about .kerlrc side effect
* Add an example of using shell `export` command in .kerlrc for build tuning
Merge pull request #56 from jj1bdx/jj1bdx-readme-diff

Update README.md