Add Support for Custom Flags

Provide a configuration option for grunt tasks to specify options
to pass to the subgrunt task as cli arguments. Option accepts these
as either a raw string or an object. Allows cli options to be chosen
on a task-specific basis instead of globally. Also add option to
choose whether the custom flags hold priority over global ones.
Use Grunt Files Array Format

Change task configuration format to use standard grunt files arrays.
This gives users more flexibility in their configuration and makes
using the plugin easier for large projects. Instead of iterating over
a 'projects' object iterate over the files array provided by grunt,
using 'file.src' as the project path. A task option, 'tasks', now
sets the subgrunt tasks. This commit does not provide backwards
compatibility but plan on readding it in the future.