Allow unmarshaling to top level maps

[oschwald]

Issue: add link to pelletier/go-toml issue here

This is related to #150, but that seems to be for map[string]interface{}, which this PR does not attempt to address.

Explanation of what this pull request does.

This allows unmarshaling to a top-level map in addition to a struct.

More detailed description of the decisions being made and the reasons why (if the patch is non-trivial).

We were porting code over from the BurntSushi TOML parser and ran into this limitation. The implementation is trivial. I am happy to make changes.

[codecov]

Codecov Report

Merging #273 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   92.02%   92.04%   +0.02%     
==========================================
  Files           9        9              
  Lines        1856     1861       +5     
==========================================
+ Hits         1708     1713       +5     
  Misses        103      103              
  Partials       45       45

Impacted Files	Coverage Δ
marshal.go	90.97% <100%> (+0.1%)	⬆️

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[pelletier]

Thank you for your pull request! Added a couple tests and merged.

[pelletier]

Feel free to submit provide additional patches and/or record issues as you transition from BurntSushi TOML. If you don't mind me asking, what are your motivations to do so?

[oschwald]

Thank you! We have found that the BurntSushi parser panics on quite a few error conditions. Some of these are fixed in various PRs, but the project seems much less actively maintained. I think BurntSushi primary does Rust nowadays.