feat: set default sorting using config option

[dundee]

closes #181

[coveralls]

Pull Request Test Coverage Report for Build 3304469701

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.06%) to 95.83%

---

Files with Coverage Reduction	New Missed Lines	%
report/export.go	1	92.02%
cmd/gdu/app/app.go	6	96.67%
tui/tui.go	16	87.91%

Totals
Change from base Build 3085653577:	0.06%
Covered Lines:	2459
Relevant Lines:	2566

---

💛 - Coveralls

[codecov]

Codecov Report

Base: 94.37% // Head: 94.48% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (92e63e1) compared to base (49e0cba).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   94.37%   94.48%   +0.10%     
==========================================
  Files          28       28              
  Lines        2507     2520      +13     
==========================================
+ Hits         2366     2381      +15     
+ Misses        103      102       -1     
+ Partials       38       37       -1     

Impacted Files	Coverage Δ
cmd/gdu/app/app.go	93.29% <100.00%> (+0.15%)	⬆️
tui/sort.go	100.00% <100.00%> (ø)
tui/tui.go	87.00% <100.00%> (+1.86%)	⬆️
report/export.go	88.12% <0.00%> (-0.63%)	⬇️

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