feat: add --[no-]shell-escape flag

[dmke]

This allows or prohibits shell command execution from within TeX files.

TODO: Verify whether latexmk -showextraoptions mentions direct support for passing either flag. latexmk(1) does not, but has examples passing it via the -pdf{lua,xe,}latex="COMMAND" options. This would complicate the construction of the latexmk command.

Fixes: #152

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.57576% with 14 lines in your changes missing coverage. Please review.

Project coverage is 71.10%. Comparing base (d0ecbbf) to head (5bdbbbb).

Files with missing lines	Patch %	Lines
cmd/texd/main.go	0.00%	11 Missing :warning:
tex/engine.go	86.36%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #153 +/- ## ========================================== - Coverage 71.32% 71.10% -0.23% ========================================== Files 34 34 Lines 1859 1886 +27 ========================================== + Hits 1326 1341 +15 - Misses 470 482 +12 Partials 63 63 ```

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