add Delete function to tree

[zenmoto]

Issue: none filed

This PR simply adds a 'Delete' function to the tree so that keys can be removed programatically.

[codecov]

Codecov Report

Merging #256 into master will increase coverage by 0.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   89.32%   89.33%   +0.01%     
==========================================
  Files           9        8       -1     
  Lines        1695     1679      -16     
==========================================
- Hits         1514     1500      -14     
+ Misses        129      126       -3     
- Partials       52       53       +1

Impacted Files	Coverage Δ
toml.go	73.3% <83.33%> (+0.52%)	⬆️
token.go

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[codecov]

Codecov Report

Merging #256 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   89.32%   89.58%   +0.26%     
==========================================
  Files           9        8       -1     
  Lines        1695     1680      -15     
==========================================
- Hits         1514     1505       -9     
+ Misses        129      124       -5     
+ Partials       52       51       -1

Impacted Files	Coverage Δ
toml.go	75.36% <100%> (+2.58%)	⬆️
token.go

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[pelletier]

Looks good! Do you mind adding a DeletePath(keys []string) (akin to GetPath and SetPath) to keep the API uniform?