Translate line and column numbers from URI handlers

The URI query string should specify line and column numbers as a user
would, starting at 1, while the Atom API starts at 0.

src/core-uri-handlers.js

@@ -1,9 +1,16 @@
+// Converts a query string parameter for a line or column number
+// to a zero-based line or column number for the Atom API.
+function getLineColNumber (numStr) {
+  const num = parseInt(numStr || 0, 10)
+  return Math.max(num - 1, 0)
+}
+
 function openFile (atom, {query}) {
   const {filename, line, column} = query
 
   atom.workspace.open(filename, {
-    initialLine: parseInt(line || 0, 10),
-    initialColumn: parseInt(column || 0, 10),
+    initialLine: getLineColNumber(line),
+    initialColumn: getLineColNumber(column),
     searchAllPanes: true
   })
 }