Set XDG_CONFIG_HOME tmux config path correctly if XDG_CONFIG_HOME in use

[kstilwell]

Intended to address #475

[tony]

If tests pass, this LGTM

[codecov-io]

Codecov Report

Merging #490 into master will increase coverage by 2.31%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   75.31%   77.63%   +2.31%     
==========================================
  Files           5        5              
  Lines         794      845      +51     
  Branches      231      274      +43     
==========================================
+ Hits          598      656      +58     
+ Misses        145      139       -6     
+ Partials       51       50       -1

Impacted Files	Coverage Δ
tmuxp/cli.py	56.83% <100%> (+0.6%)	⬆️
tmuxp/config.py	90.23% <0%> (+4.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9e1212f...9e73aa3. Read the comment docs.