add RenderInto (fixes #81)

[slimsag]

Based on #178, which should be reviewed first. Only the last commit here is part of this PR itself.

Fixes #81

[codecov-io]

Codecov Report

Merging #180 into master will increase coverage by 0.35%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   47.38%   47.74%   +0.35%     
==========================================
  Files           4        4              
  Lines         650      664      +14     
==========================================
+ Hits          308      317       +9     
- Misses        301      304       +3     
- Partials       41       43       +2

Impacted Files	Coverage Δ
dom.go	44.04% <57.14%> (+0.33%)	⬆️
domutil.go	100% <0%> (+25%)	⬆️

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[slimsag]

Given discussion in #179 the API here should probably not accept interface{}

[slimsag]

Closing since I need to update this anyway.