plots autoresize; plus other fixes

[timotheecour]

• fixes https://github.com/brentp/nim-plotly/issues/38
• partially addresses https://github.com/brentp/nim-plotly/issues/20
• some code cleanups
• quit is nasty and shd rarely be used, see https://forum.nim-lang.org/t/4042 ; now; it'll just throw on error (and give the relevant path in exception msg)

/cc @Vindaar @brentp caveat: this will ignore given size settings; auto resize is a better default than a fixed arbitrary size, but we shd honor size if provided by user; let me know if u have a quick way to patch this PR to do that

[timotheecour]

I think you might just need to set the plot div to have a %, like:

first thing I tried, but it didn't work; if you find a simpler way let me know :) will update shortly to address remaining comments

[Vindaar]

Closing in favor of #72.