Allow for tolerance to extra white space and blank lines

[simoami]

because Peg is sensitive to white space, my grammar is loaded with begin and end per expression e.g.

background = 
    (begin "Background:" end)

begin = sp*
end = (sp eol / eol) *
sp = ' '
eol = '\n'

this works with:

Background:

--or--

      Background:  

but ideally I'd prefer to keep my grammar free of white space handling around each terminal definition.

[dmajda]

Duplicate of #11.