Add support for host and command aliases

[bschonec]

It would be nice to have support for host and command aliases so that you can use the same alias multiple times in a sudoers file. eg:

Cmnd_Alias ORACLE_CMDS=/bin/ls
Host_Alias ORACLE_SERVERS=oracle1.example.com,oracle2,example.com

%admins ORACLE_SERVERS=(root) NOPASSWD: ORACLE_CMDS

[franklinkim]

see #31