Append (write) documents

[luposlip]

Right now (as of v0.4.0) the nd-db library only supports using newline delimited files as read only databases.

It should be quite simple to write new documents to the database, by appending only. In this way it will work as sort of a transaction log. To begin with probably without the notion of transaction time.

When writing data, the index should (of course) be automatically updated.

[luposlip]

The new v0.9.0 index format paves the way.