Add list of backlog issues sorted by some parameter

[filiplajszczak]

list top n issues in the backlog when top is decided either by backog order or a field on the issue.

[caseneuve]

We can find backlog issues with JQL like project = 'KEY' AND sprint is empty, then get first issue with expand="names" option, find custom field matching user provided name, and filter all found issues by values of this field.

We have 2 options:

• add more parameters to ls command
• add entirely new command, e.g. backlog (and keep ls only for sprints)

[caseneuve]

See #30