Change hook type

It makes sense to run _powerline_tmux_set_pwd only when working directory is 
changed.
Skip external_ip segment if self.ip is not defined

Ref #335
threaded: Add .update_first, leave .queries as-is unless key’s missing

Fixes #335
Closes #336
Avoid divisin by zero exception in the network load segment
Some small changes in .segments.vim
Add tests for the constantly recreated *Powerline instances
Merge remote-tracking branch 'zyx-i/threaded-misc' into develop