postgres materialized view support

Attempting to sync a database with a materialized view.

Expected result: rows in materialized view appear in index.
Actual result: document type does not appear in index.

Looks like [adaptor queries](https://github.com/appbaseio/abc/blob/2b659e2502a4c5788254924590a18bab38c5be47/adaptor/postgres/reader.go#L63) via
```
SELECT table_schema,table_name FROM information_schema.tables
```

Perhaps this is a viable alternative?
```
SELECT table_schema,table_name FROM information_schema.tables
UNION
SELECT schemaname, matviewname FROM pg_matviews
```

Version: 0.6.0
Variant: !oss
Go version: go1.10.1
OS: linux
Arch: amd64

##### Checks

- [x] I have mentioned my `abc` version in the issue, if applicable.
- [x] This issue is not a duplicate.
- [x] This issue has a description.