Fix XDG base directory supporting

[ko1nksm]

• install.sh: Install to under $HOME/.local by default
• Use $HOME/.config if not defined XDG_CONFIG_HOME

Close #24

[codecov]

Codecov Report

Merging #41 into master will increase coverage by 0.13%.
The diff coverage is 32.65%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   43.38%   43.51%   +0.13%     
==========================================
  Files          84       84              
  Lines        3082     3093      +11     
==========================================
+ Hits         1337     1346       +9     
- Misses       1745     1747       +2

Impacted Files	Coverage Δ
shellspec	0% <0%> (ø)	⬆️
install.sh	50% <100%> (ø)	⬆️
lib/libexec/shellspec.sh	100% <100%> (ø)	⬆️

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