allow user to explicitly opt out of nvm adding the source string

[spikegrobstein]

this is done by checking if the user supplies PROFILE=/dev/null when
running install.sh, the nvm_detect_profile function will not output
any strings, causing nvm_do_install to skip adding SOURCE_STR.

tests are included.

[ljharb]

What's the use case here where you want the install script but you don't want what it does?

[spikegrobstein]

We've got an automated install process for setting up workstations that uses the installer script to ensure that nvm is both installed and is the most recent version. We have our own code that gets sourced that loads nvm.sh, but install script adds it to the main shell init file, which causes it to be sourced twice.