Add inspect-def-current-value to inspect middleware

[jpmonettas]

This PR adds inspect-def-current-value that will enable cider-inspector to define a var in your current namespace with a given name and current inspector value.

Before submitting a PR make sure the following things have been done:

• The commits are consistent with our contribution guidelines
• You've added tests to cover your change(s)
• All tests are passing
• The new code is not generating reflection warnings
• You've updated the README (if adding/changing middleware)

[bbatsov]

Thanks!