Add header.left property

[grabbou]

Fixes #14

Also sorted out some eslint issues

[knowbody]

it would be great to add it to one of our examples: I checked it. You can add this:

MyProfileScreen.navigationOptions = {
  header: ({ state, setParams }) => ({
    title: `${state.params.name}'s Profile!`,

    // Render a button on the left side of the header.
    // When pressed it displays the alert message.
    left: (
      <Button
        title="Alert"
        onPress={() => alert('Welcome to React Navigation!')}
      />
    ),

    // Render a button on the right side of the header.
    // When pressed switches the screen to edit mode.
    right: (
      <Button
        title={state.params.mode === 'edit' ? 'Done' : 'Edit'}
        onPress={() => setParams({ mode: state.params.mode === 'edit' ? '' : 'edit' })}
      />
    ),
  }),
};

to SimpleStack.js example

[knowbody]

or maybe you could actually skip that example as we already show how to do it with the right one.

[knowbody]

thanks @grabbou