[ExpressionContextProvider] Make subscript values have a LOAD context

[zsol]

Fixes #318.

Summary

This brings LibCST in line with the built in ast module.

Test Plan

Changed/added tests + manually verified the following file is not touched by the remove unused imports codemod:

from a import b
b[0] = False

python -m libcst.tool codemod remove_unused_imports.RemoveUnusedImportsCommand test.py -u

[jimmylai]

This PR is awesome. It identified and fixed the issue correctly.
Given a subscript e.g.b[0] = False
The name b should have LOAD context and only the subscript b[0] has the STORE context.