Add 'blockquote' filter

Blockquote filter indents all lines from input by the specified amount of characters and wraps long lines appropriately.

Effectively, ` {{ my-variable-with-text | blockquote 4, 40 }}` transforms this:

    Building the Qt version under Linux
    ----------------------------------------------------------------------

    On Debian you need libqt4-dev, libmarble-dev, libzip-dev.
    Unfortunately the marble version in Debian stable (and possibly
    Ubuntu) appears broken and missing essential header files used in the
    current git version of Subsurface.

into this:

        Building the Qt version under Linux
        ----------------------------------------
        ------------------------------

        On Debian you need libqt4-dev, 
        libmarble-dev, libzip-dev. 
        Unfortunately the marble version in 
        Debian stable (and possibly 
        Ubuntu) appears broken and missing 
        essential header files used in the 
        current git version of Subsurface.