add whitespace control options: trim_blocks, lstrip_blocks

[jpcima]

I add these options from Jinjas2 API.

When enabled together, they can give a nicer output to conditional and loop blocks, by eliminating whitespace on lines where these blocks reside alone.
https://tedboy.github.io/jinja2/templ6.html

It's implemented on comments and statements.
Compared against Jinjas2, I obtained identical outputs.

[pantor]

Great, thank you so much!