Use kwargs in policy initializer API

[hernanat]

From #190

This commit changes the policy initializer API to take all kwargs instead of having
an optional first record arg.

[hernanat]

@palkan not sure if any documentation updates are needed, also not sure what section in the changelog to put it under as I wasn't sure if the next release would be 0.6.1 or something else.

[palkan]

not sure if any documentation updates are needed, also not sure what section in the changelog to put it under as I wasn't sure if the next release would be 0.6.1 or something else.

Please, add a change log under the ## master version.

This is a breaking change (it was planned for v1). I think, we can include it into v0.x iff:

• There is a backward compatibility (i.e., we support both positional record and record:, the latter wins if defined)
• There is a deprecation message saying that new(record) will be removed in v1, use new(record: record) instead.

All internals should use the new kwargs API.