Fix string for graph title

This function fixes a string before it is used as a title for a graph.

It is known that an underscore '_' is used in the title string to set the next character to lowercase. But, in many cases we don't need this feature, we just want to print the underscore. The correct way is to insert the '\_' pair instead of '_'. This function is doing exactly this. It can handle any number of underscores in the string.

EDITOR'S NOTE:
You can also accomplish this by setting a text object's interpreter. For example:
>> title('My_Plot','Interpreter','none')