Improve Atom's startup time (#87)

* Improve Atom's startup time

Before this change, activation was done on Atom startup, whether or not
you actually had any shell scripts open.

With this change in place, we postpone activation until the Atom
Shellscript grammar's first use.

This improves startup time of my Atom by about 24ms, thus fixing one of
the top startup time offenders according to TimeCop.

For some frame of reference, I have 88 packages installed, and Timecop
lists all packages with startup times above 5ms.

package.json

@@ -8,6 +8,7 @@
   "engines": {
     "atom": ">=1.4.0 <2.0.0"
   },
+  "activationHooks": ["language-shellscript:grammar-used"],
   "scripts": {
     "test": "apm test",
     "lint": "eslint ."

spec/linter-shellcheck-spec.js

@@ -11,11 +11,18 @@ describe('The ShellCheck provider for Linter', () => {
   beforeEach(() => {
     atom.workspace.destroyActivePaneItem();
 
+    // Info about this beforeEach() implementation:
+    // https://github.com/AtomLinter/Meta/issues/15
+    const activationPromise =
+      atom.packages.activatePackage('linter-shellcheck');
+
     waitsForPromise(() =>
-      Promise.all([
-        atom.packages.activatePackage('linter-shellcheck'),
-      ])
+      atom.packages.activatePackage('language-shellscript').then(() =>
+        atom.workspace.open(cleanPath))
     );
+
+    atom.packages.triggerDeferredActivationHooks();
+    waitsForPromise(() => activationPromise);
   });
 
   it('finds nothing wrong with a valid file', () => {