Complete all keywords and builtins (#44)

* feat: complete some keywords

* fix: if type

* feat: complete all keywords and builtins

* feat: mention completion

* feat: add abbr documentation

* feat: automatically append help option when available

* feat: add links to web documentation

* feat: add docs for more builtins

* feat: explain how to add docs

* feat: add docs up to fish command

* fix: else type

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contributing
+
+To make some command listed in intellisence suggestions use either `Keyword` or
+`Builtin` TypeScript function in [`extension.ts`](./src/extension.ts).
+
+To explain options available for a command (currently it's possible just for
+builtins) add an array of objects like this:
+
+```typescript
+          Builtin("alias", "Create a function", [
+            {
+              description: "Save into your fish configuration directory",
+              long: "save",
+            },
+          ]),
+```
+
+as the third parameter. By default it's assumed that all builtins have
+`-h`|`--help` options available, but if they are not pass `false` as the last
+argument.

README.md

@@ -9,6 +9,12 @@ Add syntax highlighting, linting, code formatting and snippets for the fish shel
 
 <img src="highlighting.png" alt="syntax highlighting" width="400"/>
 
+## Completion
+
+Non-context aware completion for keywords,
+builtins and functions. Custom user functions are not shown
+in completion results.
+
 ## Code Linting
 
 Code linting uses `fish -n`.