@ignore decorator to skip timeline & mention checks

I wanted a way to skip checking the bot's timeline and mentions, which is useful for lightweight/dumb bots.

This adds an optional `@ignore` decorator for bot makers. Use `@ignore` with `on_timeline` to avoid fetching the timeline; use it with `on_mention` to avoid fetching mentions.

Example:

```python
from twitterbot import TwitterBot, ignore

class FartBot(TwitterBot):

    @ignore
    def on_mention(self, tweet, prefix):
        pass

    @ignore
    def on_timeline(self, tweet, prefix):
        pass
```