Spaceless block tag to remove unnecessary whitespace

I created a special built-in tag which removes all unnecessary whitespace inside of it. It is useful when you want to keep a readable template by splitting it into multiple lines, but template operates in inline context and whitespaces and newlines matter and are visible in the output.

Added few simple tests.

It affects only the current template, not included ones.

Alternatively, this could be also an attribute to the `<template>`, but I preferred this style because it allows more control (having to split things into sub-templates is complicated because then you have sometimes to copy helpers as well).