Ongoing fixes while battle testing Literate integration

[tlienart]

Minor

• dont show nothing
• make output of class plaintext to avoid automatic highlighting from highlight
• suppress output if last line finishes with ;
• suppress output if last line is a statement that writes to stdout
• adds LOCAL_PAGE_VARS["literate_only"] if set to true will assume that if there a literate insertion then it's the only source of code on the page and so uses literate to determine if code has changed in one shot; if it hasn't changed, freeze the code to skip eval; if it's not the only source of code, it should be set to false (but recommend that this not be the case... shouldn't blend too much)
• more tests

Major

• track literate files, process files which may call modified literate; this might only be used for minor changes but would be useful nonetheless

[codecov]

Codecov Report

Merging #264 into master will increase coverage by 0.13%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   90.89%   91.02%   +0.13%     
==========================================
  Files          30       30              
  Lines        1504     1560      +56     
==========================================
+ Hits         1367     1420      +53     
- Misses        137      140       +3

Impacted Files	Coverage Δ
src/JuDoc.jl	100% <ø> (ø)	⬆️
src/converter/md.jl	100% <100%> (+1.66%)	⬆️
src/converter/md_blocks.jl	98.4% <100%> (+0.8%)	⬆️
src/converter/lx.jl	96.52% <100%> (+0.31%)	⬆️
src/converter/lx_simple.jl	90.9% <100%> (+2.84%)	⬆️
src/misc_utils.jl	100% <100%> (ø)	⬆️
src/parser/html_blocks.jl	100% <100%> (+3.22%)	⬆️
src/jd_paths.jl	100% <100%> (ø)	⬆️
src/jd_vars.jl	100% <100%> (ø)	⬆️
src/misc_html.jl	100% <100%> (ø)	⬆️
... and 12 more

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