added support for configuration files in the following locations:

- ~/.tmux.conf and ~/.tmux.conf.local
- $XDG_CONFIG_HOME/tmux/tmux.conf and $XDG_CONFIG_HOME/tmux/tmux.conf.local
- ~/.config/tmux/tmux.conf and ~/.config/tmux/tmux.conf.local

location of configuration files is determined by the following environment variables:
- TMUX_CONF
- TMUX_CONF_LOCAL

the TMUX_PROGRAM environment variable contains the path to the tmux executable

resolves gpakosz#200, resolves gpakosz#221, resolves gpakosz#439, resolves gpakosz#586, resolves gpakosz#624