Add ULID Primary Key Support

[morehawes]

Adds support for ULIDs (Universally Unique Lexicographically Sortable Identifiers) by allowing CHAR(26) as primary key.

This can currently be achieved using VARCHAR(26), but this adds CHAR support.

Before change:

Schema::create('users', function (Blueprint $table) {
  // ...
  $table->increments('id');
  // ...
}

After change:

Schema::create('users', function (Blueprint $table) {
  // ...
  $table->char('id', 26)->primary();
  // ...
}

Related to #54.