Tectonic.toml: preamble, index and postamble

[jeffa5]

This enables the use of preamble, index and postamble keys in an output.
These get built together to form the input buffer and default to the
current defaults too.

There isn't any fancy checking here as we defer it to the latex engine.
In theory this could enable injection but I'm not aware that would be a
concern at the moment.

Fixes #745

[codecov]

Codecov Report

Merging #746 (b91844a) into master (f325a7a) will increase coverage by 0.05%.
The diff coverage is 74.28%.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
+ Coverage   46.80%   46.86%   +0.05%     
==========================================
  Files         118      118              
  Lines       53707    53742      +35     
==========================================
+ Hits        25139    25185      +46     
+ Misses      28568    28557      -11     

Impacted Files	Coverage Δ
src/errors.rs	60.60% <ø> (ø)
src/document.rs	69.74% <74.28%> (+0.67%)	⬆️
tectonic/xetex-xetex0.c	79.01% <0.00%> (+0.17%)	⬆️

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[pkgw]

Thank you! At first blush this looks good, and I really appreciate you adding some test coverage.

Would you be able to go even a step farther and document the new settings in the reference? docs/src/ref/tectonic-toml.md would be the file.

I'm pretty oversubscribed this week so it might be a bit before I can do a full review. Please ping me if it's been a little while.

[jeffa5]

Thanks for taking a quick look.

I've now added some documentation, happy for comments :)

[pkgw]

Sorry, this is taking me even longer than expected to get to. If anyone else wants to do a code review they are more than welcome to offer their opinion!

[pkgw]

After such a long wait ... this looks great! Thank you! My apologies for taking so long to review and merge this .. if it's any consolation, it's not like I've been working on anything else Tectonic-related in the meantime ...