add [gemset] option to gemset active command (like rbenv local command)

I offen forget '.rbenv-gemsets' filename. so implement this patche.

if run "gemset active <gemset>", create .rbenv-gemsets and put <gemset> in it.
if "gemset active", show active gemsets (same as it was)

#### usage
````sh
$ rbenv gemset active FOO
activate FOO gemset (/Users/t-koshiba/dev/rbenv-gemset/.rbenv-gemsets)
$ rbenv gemset active
FOO
$ ls .rbenv-gemsets 
.rbenv-gemsets
````