wp-now: Mount SQLite database directory from wp-content stored in .wp-now

[wojtekn]

In this PR, I propose to store the SQLite database directory in the wp-content/ directory inside the hidden utility ~/.wp-now directory and mount it in the project's document root.

It fixes: #313

[danielbachhuber]

I verified this worked as expected by:

• Running wp-now start in a 'plugin' directory: no folders created.
• Running wp-now start in a 'wp-content' directory: database and plugins/sqlite-database-integration directories created, but they are empty (as expected).
• Running wp-now start in an 'index' directory: no folders created.