Add appendSpace option

Some HTML formatters remove trailing spaces from HTML source files.  This may introduced incorrect HTML code after compiling.  On the other hand, an extra space at the end of each line do not create much problem for the browsers.

For example, in the template contain the following lines (without trailing spaces)

    <input type='text'
        data-slider-min=0
        data-slider-max=1>

After compilation, the HTML become

    <input type='text'data-slider-min=0data-slider-max=1>

But if trailing space is added before compiling, the HTML is correct

    <input type='text' data-slider-min=0 data-slider-max=1>