fix(#94): Add to config IMAP prefix

Adding the IMAP prefix/root to the config will make easier to find
the Trash/Sent/Spam folders.

Default value is INBOX (courier, cyrus servers)
fix(#94): Add config options for different folders: Trash, Draft

Also rename the IMAP_SENTFOLDER and IMAP_ROOT_FOLDER. Normalize
as IMAP_FOLDER_<TYPE>.