Support indent_size = 0

The tests were derived by running `editorconfig index.js` with an
`.editorconfig` file containing:

    [*]
    indent_size = 0

and converting the output to JSON.

Closes #1

index.js

@@ -18,7 +18,7 @@ function editorConfigToPrettier(editorConfig) {
     editorConfig.indent_size !== "tab"
   ) {
     result.tabWidth = editorConfig.indent_size;
-  } else if (tabWidth) {
+  } else if (tabWidth !== undefined) {
     result.tabWidth = tabWidth;
   }
 

test.js

@@ -43,3 +43,13 @@ assert.deepStrictEqual(
     printWidth: 100
   }
 );
+
+assert.deepStrictEqual(
+  editorconfigToPrettier({
+    tab_width: 0,
+    indent_size: 0
+  }),
+  {
+    tabWidth: 0
+  }
+);