Add create_and_get_feed function

[kattni]

Adds a create_and_get_feed function to the library that attempts to get the feed, and if the feed does not exist, it creates and then gets the feed. Eliminates the need for the try/except in examples that try to create a feed.

Included an example of how it might be used, using the CPU temperature, which works on most CP-compatible boards.

[kattni]

@jerryneedell @brentru Thanks so much!