Check additional directories for write access via 'brew doctor' 

Check that we can write to HOMEBREW_PREFIX/bin, HOMEBREW_PREFIX/opt and HOMEBREW_CELLAR.
```
Warning: Some directories in /usr/local/bin aren't writable.
This can happen if you "sudo make install" software that isn't managed
by Homebrew. If a brew tries to add locale information to one of these
directories, then the install will fail during the link step.
```

I don't think this message is quite correct as the problem isn't subdirectories but the directory itself and files. Better to use a message more like that for the cellar below.