add soft reload option

[steebchen]

It would be nice to be able to use a soft reload. I am using hashes in the query string in my filenames – as soon as the page reloads the hash from a file would have already been changed so it only reloads the edited file and loads all other files from the cache. Currently, browser-sync forces to reload everything which makes everything slightly slower.

The line which should be extended or changed is the following: https://github.com/BrowserSync/browser-sync-client/blob/master/lib/browser.utils.js#L141

I'd be happy to send a PR if this would be a configuration feature you'd like to add.