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

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

tex/engine_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestEngine_LatexmkCmd(t *testing.T) {
-	t.Cleanup(func() { SetShellEscaping(RestrictedShellEscape) })
+	t.Parallel()
 
 	const mainInput = "test.tex"