Parse headline title as text, update transform and render

[schoettl]

• [x] Parse headline title as text
• [x] First, merge #40 and #42
• [x] Extract tags from headline in transform step
• [x] Join consecutive :text-normal in text
• [x] Join consecutive :text-normal in headline
• [x] Write render function for :text
• [x] Update README samples

[schoettl]

Hi @branch14 @munen

I'd like to merge this soon to be able to continue with other pending branches.

There is a lot of room for tests (transformation and rendering) but I'd rather starting to introduce more full-roundtrip tests.

We made a pretty big step with this branch (see updates in README).

Also, I refactored and introduced insta/transform. I think it makes a lot transformations much simpler and less verbose because this function handles the buttom-up tree walk and update/replace for us.