Env variables should clear other output mode's variables

[gene1wood]

If you run

$(maws --output envvar)

then later in the same terminal you run

$(maws --output shared)

Your environment then has a combination of variables set that won't be using the shared credentials.

In addition to outputing the statements that add new environment variables, we should also delete any potentially conflicting environment variables

[gene1wood]

Fixed in #185