fix prependTo and allow regex to be passed in

[jackbrown]

Fixes an issue we are having where the filenames passed in to prependTo are the only ones that don't get the script prepended. We are code splitting, so this ends up being quite a few files.

Also added the option to pass in a RegEx. This makes it easier to target a single file when multiple files have been split from the same output filename.

For example:

const outputFilenames = ['bundle.web.js', '0.bundle.web.js', '1.bundle.web.js'];

const matches  = outputFilenames.filter(filename => filename.indexOf('bundle.web.js') > -1);
// [ 'bundle.web.js', '0.bundle.web.js', '1.bundle.web.js' ]