Add Str.JoinList

[ymh199478]

/// <summary>
/// Returns all sequential combination of the given array.
/// </summary>
/// <remarks>
/// v[0] = "hello"
/// v[1] = "world"
/// var result = Str.JoinList(v, "/"); 
/// result[0] == "hello";
/// result[1] == "hello/world";
/// </remarks>
/// <param name="source">The source array.</param>
/// <param name="separator">The separator.</param>
/// <returns>The sequential combination array.</returns>
public static string[] JoinList(string[] source, string separator = null)