Add handler to parse source and transform target paths

[xsawo]

This PR introduces two new options (handler functions) to parse source and transform target paths. We use a template engine where the paths to the resources are generate at runtime and therefore it's necessary to modify the paths before processing. I've added a test to show a possible use-case.

options.parseSourcePath

Type: Function Return: The path to the source file

Function to parse the source path out of a script or style element.

The function gets the following arguments:

• tag (String): The html script or style tag
• type: (String): The type e.g. js, css

options.transformTargetPath

Type: Function Return: The transformed path to the target file

Function to transform the target file path.

The function gets the following arguments:

• target (String): The path to the target file
• type: (String): The type e.g. js, css

[xsawo]

This also fix issue #10