TODOs

Add --ignore-attributes-for-classes option. When visiting such a class, mark all its attributes as used. Fixes (partly):
- #309
- #264
- #249
- #315 Use these as test cases.
Honor (speaking) pylint error codes (e.g., # pylint: disable=unused-import): unused-import, unused-variable, unused-argument, possibly-unused-variable and unreachable-code. See https://github.com/janjur/readable-pylint-messages#unused-import.

Non-TODOs

Ignore hidden files and directories (might be unexpected, use --exclude instead).
Use Assign instead of Name AST nodes for estimating the size of assignments (KISS).
Only count lines for unused code by storing a function get_size in Item for computing the size on demand. This is 1.5 times as slow as computing no sizes.
Compute sizes on demand. Storing nodes increases memory usage from ~120 MiB to ~580 MiB for tensorflow's Python code.
Detect unreachable code for ast.Assert (assert False is common idiom for aborting rogue code).
Detect superfluous expressions like a <= b, 42, foo and bar occurring outside of a statement (hard to detect if code is unneeded).
Detect that body of if foo: is unreachable if foo is only assigned "false" values (complicated: e.g., foo = []; foo.append(1); if foo: ...).
Use coverage.py to detect false-positives (#109). Workflow too complicated.
Ignore some decorators by default: @app.route, @cli.command.
Ignore functions in conftest.py files that start with "pytest_".