Changing the cache directory

## Library version

*What's the installed library version ? Check with `instalooter --version`*:

```
instalooter v2.0.1
```

## Environment

*Describe here your environment, including:*

* *OS* Linux kernel version – 4.9.85-38.58.amzn1.x86_64 (AWS amzn-ami-hvm-2017.03.1.20170812-x86_64-gp2)
* *Python version* 3.6

## Error description - installation

I am using InstaLooter as a library (rather than run from the command line). Testing locally works fine however I am trying to deploy to AWS and running into an issue where InstaLooter tries to cache some files in a location that is read only.

I did not see any way in the documentation (or a quick look through the source) to configure the path to the cache folder. How can I change the location so that it can be set to a writable directory?

## Error description - runtime

> unable to create /home/sbx_user1059/.cache/instalooter/2.0.1 ([Errno 30] Read-only file system: '/home/sbx_user1059'): CreateFailed

*Are you using the API ? If so, include a small snippet that can be used to re-raise the
error:*

Exception is throw immediately after trying to instantiate InstaLooter:

```python
from instalooter.looters import ProfileLooter
looter = ProfileLooter(profile)
```

### Expected behaviour

The cache directory can be modified since the default location is read only.

### Actual behaviour

InstaLooter fails to create a ProfileLooter since it cannot write the cache directory.