Validate feed key against IO naming scheme

[brentru]

Addresses #67

This library now performs validation for any method which access feed keys (via https://learn.adafruit.com/naming-things-in-adafruit-io/the-two-feed-identifiers):

1. The only characters we permit are lower case English letters ("a" to "z"), numbers, and dash ("-").
2. The name must be a maximum length of 128 characters.

Test case #1
io.publish("brubell/feeds/wrongfeedkeyformat", 1) yields:

Traceback (most recent call last):
  File "code.py", line 94, in <module>
  File "/lib/adafruit_io/adafruit_io.py", line 414, in publish
  File "/lib/adafruit_io/adafruit_io.py", line 43, in validate_feed_key
TypeError: Feed key must contain lower case English letters, numbers, and dash.

Test case #2
io.publish("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", 1) yields:

Traceback (most recent call last):
  File "code.py", line 94, in <module>
  File "/lib/adafruit_io/adafruit_io.py", line 414, in publish
  File "/lib/adafruit_io/adafruit_io.py", line 41, in validate_feed_key
ValueError: Feed key must be less than 128 characters.

Tested on Adafruit CircuitPython 6.2.0-beta.2 on 2021-02-11; Adafruit MagTag with ESP32S2

[brentru]

cc @askpatrickw This should address the issue you had.