Palindrome Exercise

New exercise. Hopefully not too similiar to other exercises. The code should be pretty self-explanatory.
As I've mentioned in another exercise: I wonder on what level we should put things like this. This is clearly educational and pedagogical: looping over each char in a string and adding it to a new string only if it's a letter, whicespace or hyphen. 

In another exercise I suggested using `String.replace()` to remove these characters. What do you think?

First, the functional programmer in me wanted to use something like:
```python
def clean_string(string):
	"""Removes non-letters from string leaving only ascii letters (swedish characters removed),
	whitespace and hyphens. Returns the clean string.
	"""
	chars = list(string.lower())
	return "".join(filter(lambda c: c.isalpha() or c.isspace() or c == '-', chars))
```
But why would I, since `String.replace` is so much simpler?