Support literal multiline marshal

[pelletier]

Use struct tag multiline:"true" literal:"true" to enable it.

[codecov]

Codecov Report

Merging #485 (0f91e71) into master (b59c12a) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #485      +/-   ##
==========================================
+ Coverage   92.84%   92.88%   +0.03%     
==========================================
  Files          12       12              
  Lines        2293     2305      +12     
==========================================
+ Hits         2129     2141      +12     
  Misses        124      124              
  Partials       40       40              

Impacted Files	Coverage Δ
marshal.go	95.93% <100.00%> (+0.01%)	⬆️
toml.go	73.33% <100.00%> (+0.21%)	⬆️
tomltree_write.go	94.88% <100.00%> (+0.11%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b59c12a...0f91e71. Read the comment docs.