Added --host_identifier option

A potential solution for #230

Adds the `--host_identifier` command line option, which allows you to specify a way of identifying the host.

`--host_identifier=hostname`: use the hostname as the identifier (default behavior)
`--host_identifier=uuid`: use a uuid to identify the host. On OSX, this will attempt to use the uuid from `gethostuuid`. On linux, it will just generate a new uuid and store it in rocksdb.

If anything fails or an invalid argument is given, OsQuery will default to using the hostname.