Multi table import

[craigds]

This changes the init and import commands so they can import multiple tables at once.

Further, it changes the usage so that:

• there's an --all-tables option
• tables are now positional arguments for the import command both commands
• any table can be renamed during import using source_table:dest_table
• the existing optional (poorly named) DIRECTORY argument to import has been removed. It renamed tables, so is obsoleted by the above.
• tables cannot be specified in the init --import command - it always imports all tables.
• the existing optional DIRECTORY argument to the init has been moved to a --path PATH option. This was required to make table names unambiguous in that command. The command raises a helpful error if you attempt to init with a table name without passing --import, because that's now clearly a mis-usage.

todo:

tests:

• add a test for multiple table import

Fixes #92 .

[craigds]

In the latest commit, I changed tack after some offline feedback:

remove just the option to specify tables from sno init --import. ie, you can import in your init command, but you can only import a whole database and you can't rename tables. if you need more fine-grained control you have to use sno init and sno import seperately.

@rcoup agreed:

Agree that any advanced behaviours should use init + import separately.

The latest commit implements that behaviour - it's not possible to specify tables for the sno init --import command, and thus it was okay to reinstate the DIRECTORY argument for sno init as it no longer conflicted with positional table arguments.