don't make use of pushd and popd as they are not universal
made NVM_DIR and NVM_SOURCE configurable using environment variables
replaced undocumented option for specifying the profile as first argument to specifying it as a environment variable (eg: PROFILE=~/.myprofile ./install-gitless.sh)
also; fixed indentation and added .zshrc as possible profile
Updated source string, use hard path as NVM is not always installed in `$HOME/.nvm`
updated output without special characters (tab; \t)
Omit duplicate 'finish line'
normalized indentation
updated README