Add "total time today" properties and updated events

[JasonWei512]

Add these members to WakaTime class:

/// <summary>
/// A string like "3 hrs 42 mins".
/// </summary>
public string TotalTimeToday { get; }

/// <summary>
/// A string like "3 hrs 4 mins Coding, 20 mins Building, 18 mins Debugging".
/// </summary>
public string TotalTimeTodayDetailed { get; }

/// <summary>
/// Fired when <see cref="TotalTimeToday"/> and <see cref="TotalTimeTodayDetailed"/> are updated.
/// </summary>
public event EventHandler<TotalTimeTodayUpdatedEventArgs> TotalTimeTodayUpdated;

This is for wakatime/visualstudio-wakatime#144.