Check capacity against multiple metros, facilities, and plans at once

[displague]

fixes #158

Deprecations:

• --plan becomes --plans (-P is backwards compatible, --plan adds to --plans args)
• --metro becomes --metros (-m is backwards compatible, --metro adds to --metros args)
• --facility becomes --facilities (-f is backwards compatible, --facility adds to --facilities args)

$ go run ./cmd/metal/main.go  capacity check -m sv,ny -P c3.small,c3.medium -q 10 
+-------+-----------+----------+--------------+
| METRO |   PLAN    | QUANTITY | AVAILABILITY |
+-------+-----------+----------+--------------+
| sv    | c3.small  | 10       | true         |
| sv    | c3.medium | 10       | true         |
| ny    | c3.small  | 10       | false        |
| ny    | c3.medium | 10       | true         |
+-------+-----------+----------+--------------+

[cprivitere]

/lgtm