add scheme argument to post_xx, delete_xx, list_xx, etc.?

e.g.,

``` r
post_site(mysites, scheme='mda_streams_dev')
```

would set the scheme to 'mda_streams_dev' before posting, post, then reset the scheme to whatever it was before the call.

pro: fewer accidental changes to the real `mda_streams` files while testing functions
pro: wouldn't be that hard to implement
con: would require time & changes to many files