Add fish shell vi-mode support
add _POWERLINE_DEFAULT_MODE detection
Set default mode in another function
Wait until fish_update_completions finishes
Make COMMAND_PATTERN consistent with other arguments
Test mode support in fish

WARNING: Requires development version of fish. Since fish is not tested in
travis it is probably OK.
Add a directory that will make fish_update_completions not launch
Merge pull request #1012 from ZyX-I/fish-vi-mode

Add fish vi mode support

Closes #879
Fixes #878