JS: add persistent storage taint steps

[ghost]

This adds a simple, extensible flow step between writes and reads to persistent storage. The step is instantiated with cookie and webstorage models.

Potential extensions:

• extend this to include steps for database accesses and JSON web tokens
• allow flow between writes and reads for mixed cookie libraries
• model server-side cookie libraries

Performance and results are unchanged.

[xiemaisi]

LGTM, just some minor nits.