Provide a warning for invalid User Agents

[brenapp]

Reddit has specific guidelines for User Agents, it might be wise to soft enforce these rules with a warning, something like so:

[Snoowrap] Your User Agent does not follow Reddit Guidelines, please consider changing it

For reference, here are the guidelines:

<platform>:<app ID>:<version string> (by /u/<reddit username>)

[not-an-aardvark]

This seems like it might be a good idea. I have a few potential concerns:

• It's only possible to disable warnings after constructing a snoowrap instance, and the user agent needs to be provided when constructing it, so it wouldn't be possible to disable this warning. This seems like it could be annoying, so we would need to figure out a way around that.
• I'm not sure how many people actually follow that guideline precisely. Arguably they probably should be following it, but it seems like it might be annoying to start emitting warnings when it has worked fine before.