Support for the login option is very minimal, but the `XONSH_LOGIN`
variable is set in this case so that scripts may take an appropriate
action.
Fixes#517
I wouldn't normally do something like this but issue #487 brought to
my attention the fact that too many of the python modules don't have
an encoding comment and of those that do there is a lot of pointless
inconsistency in the style of the comment.