Add EnrichingProxy to run enrichment during !Lookups

[akx]

It seems to work... Got ideas for better test cases?

Fixes #15

[japsu]

@akx in your opinion, should this work (it doesn't)?

emrichen/tests/test_lookup.py

Lines 102 to 113 in ba1b01d

	@pytest.mark.xfail
	def test_recursive_data_structure():
	template = Template.parse('''
	!Defaults
	x:
	y: 5
	x: !Var x
	---
	five: !Lookup x.y
	also_five: !Lookup x.x.x.x.x.y
	''')
	assert template.enrich({}) == [{'five': 5, 'also_five': 5}]

[japsu]

Regardless of the new failing test I added, LGTM & make a new issue about recursive data structures.

[japsu]

Also once this is merged, let's release 0.3.0. We've added new tags and this change may introduce subtle changes in the degree of laziness, so incrementing the minor is warranted (would probably be 2.0.0 if we were already 1.x).

[japsu]

See examples/peano.yml, uncomment five.

five:
- - []
two:
- - []

🤔⁉️

[akx]

@akx in your opinion, should this work (it doesn't)?

As far as correctness goes, it should, shouldn't it?