introduce support for multiple profiles

This should allow connections to several github instances, by binding
dynamically the `gh-profile-current-profile' variable
make use of the new profile feature
adding :profile field

also removing useless definition of :base
new function gh-profile-completing-read
version 0.7.0
add customizable filter for usernames

Under some conditions, github maps login to something slightly different
in URLs.
bumping version to 0.7.1
fix README packaging