Adding filter plugin API

[frsyuki]

We want to apply conversion or filtering to records when:

• hash: applying a hash function to sensitive columns (e.g. account_id, email)
• grep: skipping a record if it matches certain condition (e.g. skip access logs if they're internal callback requests)
• convert column type: convert created_at:string column to timestamp
• skip column: convert comment field to nil if it's too long
• remove column: remove certain columns (e.g. remove "password" column)
• join: joining data with other data stores. (e.g. join email, age, and other user information to user_id)

[frsyuki]

Probably, configuration file is like this:

in:
  type: ...
filters:
  - type: filter_a
    key: value
  - type: filter_b
    key: value
out:
  type: ...

[frsyuki]

Implemented by #49