Refactor App#check_all_X => App#check_all .

Rename the following conceptually-similar config keys:

- `imap_mailboxes`   => `imap_filters`
- `ftp_dirs`         => `ftp_filters`
- `amzn_s3_prefixes` => `amzn_s3_filters`
- `goog_cs_prefixes` => `goog_cs_filters`
- `rax_cf_prefixes`  => `rax_cf_filters`

Add `App#check_all`, removing the following:

- `App#check_all_imap`
- `App#check_all_ftp`
- `App#check_all_amzn_s3`
- `App#check_all_goog_cs`
- `App#check_all_rax_cf`

This greatly reduces almost-duplication within `App`.