Auto reshim

### Steps to reproduce

Install any binaries with most plugins, including rust, python.

According to https://github.com/code-lever/asdf-rust/issues/14#issuecomment-438029075 , nodejs does a reshim sometimes.

### Expected behavior

reshim should occur automatically whenever possible.

### Actual behavior

Users are regularly wondering why the binaries they have installed are not available.

One very easy way to detect a reshim is needed is for plugins to expose a list of filenames which when modified indicate a reshim is needed.

Typically, the plugins would provide a list of lock filenames which may appear in the source tree.

A slightly more complicated approach is for plugins to expose a list of directories in the plugin installs area which are where new binaries will appear, and should be auto-reshimmed.

Start by tracing the existing reshim behavior and the plugin interface used by asdf. Compare the proposed lock-file detection with monitoring plugin install directories, then define how automatic reshim triggering should be exposed and tested for plugin-provided binaries.

Written by the indexing model from the issue text.