feature request: Support exclude(ignore) using .riceignore

Similar issues: #80 #37

go.rice does not support excluding files and this result in huge binary if you have folders like `node_modules`, and due to the way rice find boxes (related #47 ), it's hard to specify different box location for same box during development and production. So similar to git and docker, which has `.gitignore` and `.dockeringore`, a `.riceignore` file can be introduced to make rice more flexible without
changing how rice find boxes.

The [implementation](https://github.com/docker/docker/tree/master/builder/dockerignore) in docker for `dockerignore` can be a good start, also there is a [dockerignore library](https://github.com/codeskyblue/dockerignore)

I have already tried it in my own [fork](https://github.com/at15/go.rice/tree/feature/ignore) in [this commit](https://github.com/at15/go.rice/commit/a21c2dfc0680aa0eb3db0ec200e988fef45d2f34). It works for append zip only, and use the library mentioned above, it works for [this small project](https://github.com/dyweb/Ayi/commit/25503a697fb6f849099dea782c008f517ebbc52d). Though the behavior is not exactly same as `gitingore` and only `.riceignore` file at box root is supported.

I can make a pull request if go.rice is still under development. Thanks~