There are several issues with this method, such as #141, #112 probably
some others, which says there are problem installing plugins.
The problme is that `sourced_files` method doesn't always correctly
identifies sources files, so plugin settings defined in those
configuration files would not be loaded.
This change fixes this by doing next things:
* excluding `tmux source` command, which could present in config
* greping all `source` lines
* awk command is left original
* using `eval`, so that globed files `source *.conf` would be loaded
as well
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