### What it does
Checks for usages of `Iterator::flat_map()` where `filter_map()` could be
used instead.

### Why is this bad?
When applicable, `filter_map()` is more clear since it shows that
`Option` is used to produce 0 or 1 items.

### Example
```
let nums: Vec<i32> = ["1", "2", "whee!"].iter().flat_map(|x| x.parse().ok()).collect();
```
Use instead:
```
let nums: Vec<i32> = ["1", "2", "whee!"].iter().filter_map(|x| x.parse().ok()).collect();
```