Obtain tmux colors and dividers from the configuration

No support for `--theme_option` or `--config`: I think it is needed to use some variable *other* then `POWERLINE_COMMAND` to specify configuration overrides both to `powerline-config` and `powerline`. Possibilities:

1. Share code in `powerline/commands/*.py` and call both `POWERLINE_CONFIG_COMMAND` and `POWERLINE_COMMAND` like `$POWERLINE_…COMMAND $POWERLINE_COMMAND_ARGS`.
2. Use environment variables like those already used in zsh/zpython to specify configuration.
3. Use `$POWERLINE_COMMAND --setup` to set up tmux, not `$POWERLINE_CONFIG_COMMAND tmux setenv`.