Make bisclique cache location more configurable

Currently a user can override the cache location, but it's pretty inelegant
```python
import os

import minorminer.busclique

def myfileloc(version=None):
dir_path = os.path.dirname(os.path.realpath(__file__))
return os.path.join(dir_path, "clique_cache")
minorminer.busclique.busgraph_cache.cache_rootdir = myfileloc
```
it would be nice to have an official API for specifying the cache location