Expose way to configure cache directory

[roeierez]

Right now the cache manager uses the temp directory which means the OS can delete it any time.
Some apps can't live with this and the solution is simple to add a way in the CacheManager to configure the cache directory.
I can make a PR to solve this issue, will it be relevant?

[renefloor]

In 0.3.0-alpha.2 you can set the directory in which the files are stored.