Fix keyvalueOptions to process braces and backslahses better

[dpvc]

This PR fixes several issues with the keyvalueOptions() utility's handling of braces and control sequences, and adds an extra parameter that controls whether it should do LaTeX3-style processing, or traditional processing.

The current implementation doesn't handle some control sequences properly, in particular, \{, so,

keyvalueOptions('x = \\{');

with throw an error about a missing closing brace or extra open brace. This PR fixes that issue with the addition of case '\\' in the readValue() function.

The current implementation doesn't always handle braces properly. For example

keyvalueOptions('a = {{x}y}');

returns { a: "x}" } rather than { a: "{x}y"}. This is due to thinking that there are two outer sets of braces to be removed, rather than only one (start isn't decremented where it should be, so the if (start > brace) { start = brace } is moved to a location that takes care of that.

Finally, the current implementation doesn't remove an outer set of braces for the entire string, as it should. That is,

keyvalueOptions('{x = y}');

returns { "x = y": true } rather than { x: "y" }, as expected. This is corrected via the dropBraces flag that allows one level of bracing to be removed.

In addition to these changes, we add a flag to keyvalueOptions() that determines whether the LaTeX3 rules are to be used, or the traditional LaTeX2 rules, for parsing the key-value pairs. The difference is that traditional parsing removes any number of outermost braces, while LaTeX3 only removes one. So

keyvalueOptions('x = {{y}}');

would return { x: "y" } using traditional rules, while

keyvalueOptions('x = {{y}}', null, false, true);

would return { x: "{y}" } using LaTeX3 rules (the final true parameter controls this).

LaTeX3 key-value parsing has the ability to check types of arguments and report errors, but that has not been implemented here. It might be worth doing so in the future.

[zorkow]

lgtm.