Add .dropboxignore directory to exclude folders without using selective sync

@Jasper - Also .vagrant folders, composer vendor dir, ruby gems, etc.—almost any language has folders with giant dependency trees, and many tools (Docker, Vagrant, etc.) create one or more hidden dirs with either giant files or hundreds of small files.

Every time I set up a new project inside my Dropbox, I have to remember to quickly set up selective sync options—on all my computers—to exclude said folders, otherwise I'll notice that gigabytes of bandwidth or tons of CPU have been consumed later, then I'll have to rebuild the project after fixing up selective sync options.

I'm amazed this isn't higher on Dropbox's internal radar, for the simple fact of dogfooding—surely some of the Dropbox internal devs have run into this issue?