Allow empty messages in comments.

upstream PR: https://github.com/spf13/cobra/pull/801

Some bug trackers, like Launchpad, allow messages to be empty (when adding a
file to the bug, for instance).

Display of CJK contents are supported. Adding CJK tags are problematic.

Improve termui paging

See https://github.com/MichaelMure/git-bug/pull/83 for the rationale

After moving to the previous page, users probably want to have their cursor on
the last entry of the page. When moving to the next page, they probably want it
to be on the first entry.

Rather than using 'h' or 'l' to load the previous or next page, allow users to
do this automatically when going up or down the list with 'k' or 'j'. This is
the default behaviour in mutt, for instance.

WIP: Initial Launchpad bridge.

This a just a preview. Not all features are expected to work.

tests: configure user ident in repos used by tests

Some git operations require the user to have an identity configured and
will exit with failure if none is set (or if git can't guess it). As a
direct consequence of this, the test suite may fail depending on the
user local configuration.

The error itself is justified as regular users *should* configure their
identity themselves. However, when building in chrooted environments
it's unlikely the git identity will be set making the test suite fail
unnecessarily.

To prevent such unnecessary failures, let's make a dummy identity for
repos created and used by the test suite.

README.md: Add developer-specific information.

Cloning a Go project is a weird thing for developers unfamiliar with the
language.

This closes #77.

Cleaning lock files on Interruption