Adding filter transformer and tests.
Updated CL according to comments.
Remove transpose_transformer.dart from charts.dart, it will be checked in the next CL.
merge charts.dart

merge charts.dart

Modification base on comments
Merge branch 'filter-transformer' of https://github.com/midoringo8823/charted into filter-transformer

Conflicts:
	lib/charts/charts.dart
	lib/charts/transformers/aggregation_transformer.dart
	lib/charts/transformers/filter_transformer.dart
	test/charts/transformers/aggregation_transformer_test.dart
	test/charts/transformers/chain_transform_test.dart
resolve merge conflicts
Merge pull request #3 from midoringo8823/filter-transformer

Adding filter transformer and tests.