This commit introduces a basic help feature. We can go to it

with the `help` command to explore and list all commands available.

Enter will also try to see if the location to be entered is an existing
Nu command, if it is it will let you inspect the command under `help`.

This provides baseline needed so we can iterate on it.

src/plugins/sum.rs

@@ -64,7 +64,9 @@ impl Sum {
 
 impl Plugin for Sum {
     fn config(&mut self) -> Result<Signature, ShellError> {
-        Ok(Signature::build("sum").filter())
+        Ok(Signature::build("sum")
+            .desc("Sum a column of values.")
+            .filter())
     }
 
     fn begin_filter(&mut self, _: CallInfo) -> Result<Vec<ReturnValue>, ShellError> {