added support for configuration files in the following locations: (3), …

…fixes gpakosz#631

- ~/.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

Oh my tmux! now relies on setting @tpm_plugins so as to not impose its opinions
on TPM and its way of discovering sourced tmux configuration tiles

do not unset '@tpm_plugins' once initial setup is done as it prevents the update
plugins binding from working

.tmux.conf

@@ -1492,9 +1492,9 @@ run 'cut -c3- "$TMUX_CONF" | sh -s _apply_configuration'
 #     fi
 #
 #     if [ "$_tmux_version" -gt 260 ]; then
-#       tmux set -gu '@tpm-install' \; set -gu '@tpm-update' \; set -gu '@tpm-clean' \; set -gu '@plugin' \; set -gu '@tpm_plugins'
+#       tmux set -gu '@tpm-install' \; set -gu '@tpm-update' \; set -gu '@tpm-clean' \; set -gu '@plugin'
 #     else
-#       tmux set -g '@tpm-install' '' \; set -g '@tpm-update' '' \; set -g '@tpm-clean' '' \; set -g '@plugin' '' \; set -g '@tpm_plugins' ''
+#       tmux set -g '@tpm-install' '' \; set -g '@tpm-update' '' \; set -g '@tpm-clean' '' \; set -g '@plugin' ''
 #     fi
 #   fi
 # }