Short-hand flags (#1378)

* typo fixes

* Change signature to take in short-hand flags

* update help information

* Parse short-hand flags as their long counterparts

* lints

* Modified a couple tests to use shorthand flags

tests/plugins/core_str.rs

@@ -46,7 +46,7 @@ fn downcases() {
 
         let actual = nu!(
             cwd: dirs.test(),
-            "open sample.toml | str dependency.name --downcase | get dependency.name | echo $it"
+            "open sample.toml | str dependency.name -d | get dependency.name | echo $it"
         );
 
         assert_eq!(actual, "light");