BibTeX: Fix string definition handling and add concatenation support

[dstillman]

I suspect string definition handling has been broken since ff884aa. String definitions are properly read and substituted, but a rawValue isn't passed to processField(), which expects one for name fields:

translators/BibTeX.js

Lines 880 to 888 in 18f8d6a

	// see if there's a defined string
	if(strings[value]) value = strings[value];
	} else {
	rawValue = getFieldValue(read);
	value = unescapeBibTeX(rawValue);
	}
	if(item) {
	processField(item, field.toLowerCase(), value, rawValue);

This causes an import to hang with rawValue is undefined.

This part could be fixed with value = rawValue = strings[value].

However, it looks like we also don't support concatenation using #, and since that's likely to be used with string definitions, we should fix that at the same time.

Sample entry:

@string{TEST = "Test"}

@book(StringDefinitionTest,
author = TEST # { and Test2},
)

(I don't know when I'll be able to get to this, so it'd be great if someone else could look at this.)

[zuphilip]

Fixed in #2020.