added config file support for the most important options
Added configuration section to the README, and the hidden
--dump-config-defaults option to easily update it.
added Python virtualenv dirs to .gitignore
unify --node and --npm behaviour to allow 'latest'
minor code formatting
added remaining options that make sense in a config file
minor: code formatting
Merge pull request #85 from jhermann/nodeenv-configfile

nodeenv configfile