Jump command

When researching the bug for https://github.com/inducer/pudb/pull/90, I read in the [docs](https://docs.python.org/3/reference/datamodel.html):

> f_lineno is the current line number of the frame — writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to f_lineno.

I'm probably the last person to learn about this, but it seems like a useful thing to implement. `j` is already taken as down, but `J` is free. It's probably easy to implement, at least assuming the error reporting is good for bad jumps.

The issue points to Python's frame f_lineno documentation and the related pull request #90; start by inspecting the debugger's existing command handling and error reporting. Determine how the free J key would invoke a Jump command, then verify valid and invalid jumps behave as intended.

Written by the indexing model from the issue text.