Feature: allow specifying text in TextFrame.add_paragraph()

[kckaiwei]

Tackling this: #134

Saw this shortlist, and figured I'd tackle it since it's pretty easy. Created a new test to for creating a paragraph with text

Docs have also been updated in the quickstart and in the Text section to showcase adding with and without text.

A quick script (based off the docs) shows it adding a new paragraph (in this case, a bullet point)

prs = Presentation()

bullet_slide_layout = prs.slide_layouts[1]
slide = prs.slides.add_slide(bullet_slide_layout)
shapes = slide.shapes
body_shape = shapes.placeholders[1]
tf = body_shape.text_frame
tf.add_paragraph("THIS IS TEST")
prs.save("test.pptx")```