Random Style and Colour access

This branch contains functions that can be used to access and change the colour and style of the characters in a ```RichText::Line```, as well a corresponding interface in the ```RichText``` class itself.  It also contains an ```isolateCharacter(std::size_t)``` function in ```RichText::Line``` to accomplish the setting, and a substring function for sf::Strings in the sfe namespace. (This substring function can be replaced without too much hassle once the next version of SFML comes out with it's sf::String::substring)

The only concern I see with this setup is that the lines aren't cleaned up with adjacent, like strings concatenated, but it would be easy to implement if the duplicate formatting data is enough of a concern.