Add .dropboxignore directory to exclude folders without using selective sync

Highlights:

• Exact same file format as .gitignore and .dockerignore
• Feature could be an opt-in on the official client
• Client then ignores anything in that directory or its subdirectories that match the patterns in the .dropboxignore files up to that point.
• The above rule means that a .dropboxignore at the top of your dropbox would be applied to everything in your Dropbox, but files below that would only apply to their directories and so on.
• .dropboxignore files are synced to Dropbox unless otherwise specified?

I think this would be a really useful feature for software developers that use Dropbox to sync code between their computers because it would keep build artifacts (the various files generated when compiling code) from being synced around, which would speed up the sync process while also using less space.  This could even be profitable for Dropbox because I would bet that many of the people abusing Dropbox in this way are more likely corporate customers with unlimited plans anyway, since it is relatively untenable to do this on most personal plans.

I imagine this could also be good for other creative / professional types like video editors who don't want all of the files that something like Adobe Premier creates when it does an export.

Lastly it seems pretty straightforward to implement as these things go.  Just a couple klocs (kilo lines of code) in the desktop clients would probably do the trick.