Move all configuration to powerline-base.conf

Reasoning: for some reason it does not work after `powerline-config tmux
setcolors`.

Fixes #717
Add a way to specify configuration paths to powerline-config

One still cannot override various configuration options from the command-line.
Rename setcolors to setenv, add documentation
Refactor some calls to run_tmux_command into a new function
Do not hardcode dividers in tmux configuration, use powerline-config

Fixes #817