ENH: Added 'build_dest' option to conda rc.

`build_dest` must be a directory, where conda will place all build products.

Example condarc:
```yaml
#~/.condarc
build_dest: ~/.conda-builds
```

```
$conda build --output 
~/.conda-builds
```

This is used to help binstar build.

cc @ilanschnell @asmeurer