feat: Support marshal and unmarshal (#10)

* add Makefile
* expose all private fields
* support marshaling and unmarshaling
* upgrade go to 1.20

Add helper methods to list groups in level order

Add MatchGroup helper methods

Merge pull request #5 from relex/var-dirs-support

Support for group_vars and host_vars

Remove host from ungrouped if it belongs to any other group

Add HostsToLower and GroupsToLower methods

+ Bugfix when comment present on a line with group name

Support multiple host pattern

Support for unexplicitly defined groups

Ex.: when only `[web:children]` exists, but not `[web]`

Support for unexplicitly defined groups

Ex.: when only `[web:children]` exists, but not `[web]`

Fix case when host defined multiple times