Implement tagless and tagged htc file functionality
As discussed in dev meeting on 2018.11.29, modify hawc2_input_writer.py and htc_file.py to handle both tags (in square brackets) and 'tagless' tags. This will allow for backward compatibility when generating htc files in the old way (in the style of the preproc module) as well as giving the user a choice between tagless and tagged htc files.