Option to choose searchindex.js location

[dagnic]

When using sphinx within a flask application, all files in the _static directory are accessible. That's not the case by default for the files outside _static, unless we define specific routes.

I propose to add an option searchindex_filename to the StandaloneHTMLBuilder to be able to choose a specific location for the searchindex.js file.

EDIT: this is may be difficult since many themes (including provided by extensions) rely on the assumption that this file is in the root of the build directory.