Add text_config for DecimalNumber

[TurkeyBilly]

Added config passing for text

Motivation

Give users option to change the text config for DecimalNumber

Proposed changes

• Added text_config in decimal number config
• Removed extra init_colors() so that t2c can be passed in
• Adjustedstring_to_mob method so key word arguments can be accepted, meanwhile maintain the string_to_mob_map

Test

Code:

class Test(Scene):
    def construct(self):
        a = DecimalNumber(1.02, show_ellipsis=True)
        b = DecimalNumber(1.02, show_ellipsis=True, 
                            text_config=dict(font="仿宋", slant=ITALIC)).shift(DOWN)
        c = DecimalNumber(1.02, show_ellipsis=True, 
                            text_config=dict(font="Times New Romen", t2c={'0':RED})).shift(2*DOWN)
        self.add(a, b, c)

Result: