Add a slugify filter

I cannot think of a template I wrote that didn’t need such a filter 🙂

I usually write a very naive one myself:

repository_name:
  type: str
  help: Your repository name
  default: "[[ project_name|lower|replace('_', '-')|replace(' ', '-') ]]"

But it will break in so many cases.

As an alternative, if we don’t want to support more and more filters in copier, maybe we could think of allowing custom filters to be used. Take a look at https://github.com/mattrobenolt/jinja2-cli/pull/86, it allows this use of jinja2-cli:

jinja2 -f ansible.plugins.filter.ipaddr.ipaddr test.txt

In copier’s case, we would not be able to load the filter using its Python dotted-path, so we would have to think of another way to add it to the Jinja environment.

[baurmatt]

As described in #277: I’d like to be able to implement custom filters for e.g. generating a secret with hmac:

import binascii, hashlib, hmaci
hmac.new(binascii.unhexlify(key), msg.encode(), hashlib.sha256).hexdigest()

[pawamoy]

I recently saw that pyyaml allows to load Python objects directly: https://pyyaml.org/wiki/PyYAMLDocumentation#yaml-tags-and-python-types

The user could pass !!python/module:my_filters and we could add the all the functions defined in this module as filters in the jinja env, or something similar 🙂

This could be useful for #182 as well.