Add `src action clear-cache` subcommand

[mrnugget]

The feedback in https://gist.github.com/unknwon/b7c80d23e6b2a624e50e4073454959e0 is now another mention of how difficult it is to clear the cache for src action exec.

I discussed with @unknwon that it would be best to add a src action clear-cache command that deletes the cache (as opposed to adding a -no-cache flag to src action exec, because it's complicated to explain that it ignores cache on read but probably not on write)

[mrnugget]

On second thought: a -clear-cache flag to src action exec might be better, since it would work with the specified -cache directory.