Option to align equals signs

I would find it useful to be able to align equals signs by padding them with spaces.

For example

```
filter p (x:xs)
| p x = x : filter p xs
| otherwise = filter p xs
```

would become

```
filter p (x:xs)
| p x = x : filter p xs
| otherwise = filter p xs
```

No file, test, or entry point is named. Start by locating the formatter's handling of guarded equations and any existing configuration for formatting options. Done means an option can pad the equals signs in the shown example while preserving the existing unaligned output when the option is disabled.

Written by the indexing model from the issue text.