Add OutputFormatter, defaulting to OutputFormatter::RawThenSummary.

fm's "summary" output is mostly quite useful, in that it quickly directs
the reader's eyes to where a problem was encountered. Sometimes,
however, you need to see the output before that to understand how an
error happened where it did.

This commit gives an fm user the ability to choose from 3 outputs:
`InputThenSummary` (the new default), `SummaryOnly` (the old default),
and `InputOnly`. For example `InputThenSummary` produces output along
the lines of:

```
Raw text:
   |1
   |2
   |3
   |4
   |6
   |7
   |8
   |9
   |10

Pattern (error at line 5):
   ...
   |2
   |3
   |4
>> |5
   |6
   |7
   |8
   ...

Text (error at line 5):
   ...
   |2
   |3
   |4
>> |6
   |7
   |8
   |9
   ...
```

Note that I had to take `PartialEq` of `FMatchError`. I don't really
know why `FMatchError` impled `PartialEq`, but technically a user could
rely on it, so this commit technically breaks backwards compatibility.