feat: allow `generate` command to generate a single DBC file

[Jassob]

This commit allows a user to run

cantool generate INPUT OUTPUT-DIRECTORY

where INPUT is either a path to a DBC file. Previously it was only possible to provide INPUT as a directory containing DBC files.

N.B. When INPUT is a directory containing DBC files the generated files are created in OUTPUT-DIRECTORY relative to where they are found in INPUT.

As an example, given the following directory structure:

dbc/
   powertrain/pt.dbc
   steering/steer.dbc
   brake/brake.dbc

A cantool generate dbc/ gen/go call will generate the following structure:

gen/go/
   powertrain/pt.dbc.go
   steering/steer.dbc.go
   brake/brake.dbc.go

However, when INPUT is a single DBC file it is generated directly in OUTPUT-DIRECTORY with its basename as the stem.

cantool generate dbc/steering/steer.dbc gen/go/steering will therefore generate the gen/go/steering/steer.dbc.go.

Inspired by #292 (thanks @bertaveira!)