Add indentation setting for Encoder

[x-hgg-x]

This PR adds the possibility to set indentation when marshaling with Encoder.

Fixes #371.

[codecov]

Codecov Report

Merging #386 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
+ Coverage   95.09%   95.11%   +0.01%     
==========================================
  Files          10       10              
  Lines        2202     2209       +7     
==========================================
+ Hits         2094     2101       +7     
  Misses         68       68              
  Partials       40       40              

Impacted Files	Coverage Δ
marshal.go	94.49% <100.00%> (+0.06%)	⬆️
tomltree_write.go	95.51% <100.00%> (ø)

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[pelletier]

Nice! Thank you for adding this.