Add cached property to the response

[mtzfederico]

This PR adds a cached property to the response. This lets you check if the response was cached or not.

[UmanShahzad]

@mtzfederico thank you for the PR. I'd like to actually know what use case this serves, because currently it's not clear to me. I would imagine that if it needs to be known that a certain IP is in the cache or not, it'd be better to use the cache interface directly to do that.

[UmanShahzad]

Two nits here:

1. Add a newline between the codeblock and this line.
2. Convert the word "cached" here to an inline codeblock, i.e. "You can access the cached property..." This clearly distinguishes it as referring to something in the code.

[UmanShahzad]

What is this import?

[mtzfederico]

I find it useful for some simple statistics and for debugging, but you mention that it would be easier to use the cache interface. I am not very familiar with cachetools, it might be easier to just add some sample code for debugging to the docs.

I am not sure about that import, I just ran my tests again without it and it works. My guess is that my editor added it automatically.

[UmanShahzad]

@mtzfederico You wouldn't have to work directly with cachetools, because that's an implementation of the cache interface, which can be found at https://github.com/ipinfo/python/blob/master/ipinfo/cache/interface.py.

You can access/update the cache via dictionary-like notation, but the cache key must be gotten via ipinfo.handler_utils.cache_key. For example:

>>> import ipinfo
>>> from ipinfo.handler_utils import cache_key
>>> h = ipinfo.getHandler()
>>> ip_cache_key = cache_key('1.1.1.1')

# __contains__
>>> ip_cache_key in h.cache

# __getitem__
>>> handler.cache[ip_cache_key]

# __setitem__
>>> h.cache[ip_cache_key] = None

If this is sufficient I'd much rather we just make a PR explaining this, rather than adding a new field which is really only metadata.

[UmanShahzad]

The wrong cached was wrapped with '`'; in the example at #54 (comment) I wrapped only the first one.

[mtzfederico]

@UmanShahzad Thanks for your examples. I'm going to close this PR and create another one with the examples that you provided. Should I put it in the README.md below the "Using a different cache" section?

[UmanShahzad]

@mtzfederico that would be great, thank you. Yes you can make a new section below that as:

#### Accessing the cache directly

...