Added `git hub issue show -f <n>`

This command is for listing more information on one issue or several.
Renamed methods for printing an issue

old print_issue() is now print_issue_summary().
old print_issue_completely() is now print_issue().

As proposed in #76.
Shortened printing full issue information

This patch changes the printing of the full issue information by
composing a list of labels by joining the labels into a new field inside
`issue` and using this one.
Added issue comments printing

This patch adds the printing of the comments of the issue(s).
always print the full information on an issue, as proposed in #76
We should create a copy of `issue` (as proposed in #76)
outsourced comments from issue datastructure as proposed in #76