Add `filter` command supporting filtering by custom fields

[caseneuve]

E.g.

dzira filter --custom-field|-f 10887[:NAME] -gt|-lt|-eq VALUE

Further enhancements:

dzira filter --date|-d FORMAT -gt|-lt|-eq VALUE
dzira filter --epic|-e ID 
dzira filter --time-spent|-t FORMAT -gt|-lt|-eq VALUE
dzira filter --label NAME

Needs:

dzira meta --epic|--label|--state|--type