Add filter/endfilter keyword support, center and indent functions

[navrocky]

This MR extends Inja's template language with Jinja2-inspired filter feature:

1. Filter blocks — {% filter ... %} ... {% endfilter %}

Renders the block body to a string and passes it as the first argument to the filter expression. Any function — built-in or a user add_callback — that takes a string and returns a string works as a filter, and filters
can be chained with | and take extra arguments.

render("{% filter upper %}Hello {{ neighbour }}!{% endfilter %}", data); // "HELLO PETER!"
render("{% filter replace(\"e\", \"3\") | upper %}{{ neighbour }}{% endfilter %}", data); // "P3T3R"
render("{% filter center(11) %}name{% endfilter %}", data); // "    name   "

Filter bodies may contain {% if %}/{% for %} and other filter blocks (nesting supported).

2. New helper functions: center and indent

• center(value, width=80) — Python str.center semantics.
• indent(value, width=4, first=false, blank=false) — width accepts an int (number of spaces) or a string used verbatim as the prefix, per the Jinja2 docs. The first line and blank lines are skipped unless first / blank
  are set.

render("{{ center(neighbour, 11) }}", data);          // "   Peter   "
render("{{ indent(\"line1\\nline2\", 4) }}", data);    // "line1\n    line2"
render("{{ indent(\"line1\\nline2\", \"> \") }}", data); // "line1\n> line2"

Both work in call form and pipe form ({{ x | center(11) }}).

Closes #328