Add ability to provide imagemagick arguments in TextClip

[MohamedAbdultawab]

I'd like something like the following:

class TextClip(ImageClip):
    def __init__(self, txt=None, filename=None, size=None, color='black',
                 bg_color='transparent', fontsize=None, font='Courier',
                 stroke_color=None, stroke_width=1, method='label',
                 kerning=None, align='center', interline=None,
                 tempfilename=None, temptxt=None,
                 transparent=True, remove_temp=True,
                 print_cmd=False, extra_cmd=None):
        
        ...
        if extra_cmd is not None:
            cmd += extra_cmd

        cmd += ["%s:%s" % (method, txt),
                "-type", "truecolormatte", "PNG32:%s" % tempfilename]

This would be useful to allow expert users to do alot of things with image magick without the need for moviepy to add support all the possible image magick configurations.

I'm happy to contribute if this is something the authors are comfortable about.