Allow for override of update/remove for CMB2_Field
Fix filter to use null
Merge branch 'feature/issue-64' of https://github.com/sc0ttkclark/CMB2 into sc0ttkclark-feature/issue-64
Add more filters and documentation
update pot line-numbers
Merge branch 'sc0ttkclark-feature/issue-64'. Adds more filters for setting/getting meta values