Add initialisation-using-substring support for `Text`, similar to `Tex`

@SamuilYu is already working on this.

Since May 16, 2021.

## Enhancement proposal
Currently, `Tex` supports initialisation using multiple strings. This allows for easy access of substrings which is useful for say changing the color of a substring only. I think it would be great for `Text` to have this functionality as well.

```python3
class Example(Scene):
def construct(self):
tex = Tex("This is ", "tex (actually plain text)", "using multiple strings")
tex[0].set_color(RED)
tex[2].set_color(GREEN)
self.play(Write(tex))

text = Text("Can't initialise using multiple strings")
self.play(Write(text))
self.wait()
```