Display 'use cache' when return result using cache

[shiimaxx]

Currently, whether the plugin uses cache for return results is not contains in messages. (but, I can display it use -O option)

I included 'use cache' in a message when the plugin uses the cache, to avoid confusion.

[netmarkjp]

@shiimaxx Thanks ;)
If we aim to avoid confusion, I think it's better that we should print the datetime the cache was made too.
How do you think?

[shiimaxx]

@netmarkjp
I changed the message to contains the datetime of cache because I think it's better.

$ python check_log_ng.py -p 'ERROR' -S ./seek -l 'test.log' --cachetime=10
OK - No matches found. (use cache created at 2020-02-03T14:48:09.866695)

Current implementation uses the naive object that doesn't contain timezone information in datetime of cache.
This work on Python2 and Python3 with standard libraries only (but, trying to treat timezone in Python2 should third-party library like pytz).

[netmarkjp]

@shiimaxx Thanks a lot ☺️

seek/y2Q55o5X_gupsGXqUEpn2VDgb9k=.cache is added but should we add this file?

[shiimaxx]

@netmarkjp
Sorry, I pushed some local test files. Deleted those files and force pushed.