catmaid_login should prefer environment variables

[jefferis]

Environment variables are the preferred way of storing authentication information. They are inherited by child R processes when set in a parent and are not displayed to the screen when you look at options.

The change that needs to happen here is that catmaid_login should

1. see if it has been given explicit parameters and if so use those
2. check for a cached login
3. check for environment variables
4. check R options

Right now 1,2 happen in catmaid_login while 4 happens inside catmaid_connection. I guess 3 could also happen inside catmaid_connection.

The package docs/READMEs need to be updated to recommend environment variables. This should have the knock on effect of allowing pkgdown to run vignettes requiring authentication.