Change find_by_* to return nil rather than raise an exception

This is a breaking API change.

* Rename 2.0.0 find_by_* to fetch_by_*.
* Introduce find_by_* which calls fetch_by_* with a nil default value.
* fetch_by_* now accepts a default value as well as a default block.

You probably have code that looks like
```ruby
  if emoji = Emoji.find_by_alias($1) { nil }
```
that you can change to
```ruby
  if emoji = Emoji.find_by_alias($1)
```
If you expect an exception for missing emoji, switch to
```ruby
  emoji = Emoji.fetch_by_alias($1)
```
instead.