If tmux.conf refers other file by source-file command that uses variable,
it'll be better to expand variables into actual values.
Both single and double quotation marks are must be recognized correctly.
Add function `_get_user_tmux_conf` to helper script `plugin_functions`.
Function is searching for the users tmux configuration on multiple
places by a prioritized order.
The response is used within`_tmux_conf_contents` to read in the content
as normally.
Add new environment variable `TMUX_PLUGIN_MANAGER_CONFIG_LOCATION` which
is optional to be defined.
If so it has the highest priority to be loaded, despite if the file
exist or not.
XDG directory support has been added as well by the second priority
location at `$XDG_CONFIG_HOME/tmux/tmux.conf`.
If tmux was not yet configured to initialize the Tmux Plugin Manager,
the variable for the plugin home was not being set. This was resulting
in a `rm -rf /` which is pretty nuclear.
Fixes: https://github.com/tmux-plugins/tpm/issues/58
