metadata: support custom metadata in .dvc files

This adds support for custom metadata in outputs under `meta` key.
Also adds `--meta` flag to `add`/`import`/`import-url` that supports
`key=value` values.

tests/func/test_commit.py

@@ -51,6 +51,9 @@ def test_commit_preserve_fields(tmp_dir, dvc):
           labels:
           - label1
           - label2
+          meta:
+            key1: value1
+            key2: value2
           remote: testremote
         meta: some metadata
     """
@@ -69,6 +72,9 @@ def test_commit_preserve_fields(tmp_dir, dvc):
           labels:
           - label1
           - label2
+          meta:
+            key1: value1
+            key2: value2
           remote: testremote
           md5: acbd18db4cc2f85cedef654fccc4a4d8
           size: 3