Do not include files ignored by Git in compiled assets

Twice I nearly committed Vim swapfiles into `bindata.go`.

This automatically adds files that exist under `static/` and [match user- and project-configured ignores](https://www.kernel.org/pub/software/scm/git/docs/git-ls-files.html) to the ignore list of `go-bindata`.

```
➜ make assets 
go-bindata   -ignore=static/js/[.]app[.]js[.]swp \
                -ignore=[.]gitignore -ignore=[.]gitkeep static/...
```