Add CLI command for cache purge

[pnikolov]

The command can be:

$ silla cache-purge

This should delete all contents of the following directories:

• temp/cache
• public/*/assets/js/
• public/*/assets/css/

[sandio]

1. How is a user of the command going to distinguish between the assets' cache and the cache used to store key-value pairs? Note: If the key-value cache is using the filesystem adapter, then the above would not be a problem (key-value pairs are stored in temp/cache). However, if the key-value cache is any other adapter, then the semantics of the above command are not that clear, in my opinion.
2. What do you think about using a three-word commands silla cache purge, instead of silla cache-purge? This way other commands concerning the cache or any other component will fit intuitively.
3. Generally, what do you think about using command flags, both with a long --flag and short -f version? This can be a separate issue of discussion.

[pnikolov]

I have not thought about the cache using different cache adapters... You are right the three word syntax is better. For example:

• Purge all cache (by default)

$ silla cache purge --all

• Purge only assets cache

$ silla cache purge --assets

etc.

[pnikolov]

@kalins can you share the latest code regarding this command?