Previous implementation exits abruptly when no version is installed for
a plugin. This prevented the list command from listing the versions for
some other plugins.
This commit allows list command to continue executing even when no
version is installed for some plugins.
Example, plugin a with 1.0, b with none, and c with 2.0.
Previous implementation:
```
$ asdf list
a
1.0
b
No versions installed
```
After commit changes:
```
$ asdf list
a
1.0
b
No versions installed
c
2.0
```
- Remove plugin shims when last version is uninstalled.
- Remove shims on plugin-remove
When the latest version of a tool is uninstalled,
Remove the plugin shims (marked with metadata at #122)
Also found lots of missing tests and added them.
Closes#67
