i18n: Support concatenation in string extraction

[aduth]

The i18n utility uses static analysis to extract translated strings from code, and is therefore not able to handle complex logic. Currently, the argument to localization functions must be a simple string. While we cannot feasibly support complex logic, we can at least support string concatenation, since it is parsed into a relatively-easy-to-handle BinaryExpression type. This would be useful to allow long strings to be split across multiple lines.

AST Example:

http://astexplorer.net/#/gist/2440bf8d51046508fb754b528e054bc5/19c4c4dcace89a8d8fe42f525f95d9577212ef6d

Possible implementation inspiration:

• https://gist.github.com/aduth/fd03f78326e52f2daecf0d21bade1921#file-moize-minify-js-L8-L18
• https://github.com/Automattic/i18n-calypso/blob/2a7cbbb/cli/preprocess-xgettextjs-match.js#L73-L88