FR - allow fetching username and password

[mikegriffin]

It might be useful in some environments, where credentials are not stored locally, to allow arbitrary commands to fetch them, eg:

[mysql:client]
credential-method=fetch
user-fetch-cmd=curl foo1 | base64 --decode
passwd-fetch-cmd=curl foo2 | base64 --decode

These results should probably be stored in a memory, ie not in an environment variable or file