Invalidate cache if linter path or arguments change (#19)

[siebert]

The cache is not invalidated if the arguments (or path) of the linter program changes. To fix this I change the path where the cached data is stored. A sha256 hex hash value is appended of the linter name, which is created by hashing the linter program name and arguments.

[sk-]

Thanks @siebert for the PR. I added some comments, and you would still need to fix the e2e tests.

However the bigger question is regarding outdated cache files. As now without a readable name for the cache it's almost impossible to know which folders you could remove.

I think that an idea that incorporates your hash idea is to have a placeholder along the cache directory, something like:

cache
 - python.hash
 - python
    - file1
    - file2
 - ruby.hash
 - ruby
    - file4
    - file5

In that way whenever the hash changes you could remove the cache folder.

[siebert]

@sk- Can you review the current patch again?