support gutter by @gutter and flexgrid kwarg gutter

[hhaensel]

Support Quasar's q-gutter / q-col-gutter by adding three new flexgrid kwargs gutter, xgutter and ygutter.
Depeneding on whether the element is a container (rowor column) it chooses the correct gutter setting in class:

julia> row(gutter = :md, @gutter [
         card("Hello", sm = 12, lg = 4)
         card("World", sm = 12, md = 8)
       ]) |> prettify |> println
<div class="row col q-col-gutter-md">
    <div class="col col-sm-12 col-lg-4">
        <q-card>
            Hello
        </q-card>
    </div>
    <div class="col col-sm-12 col-md-8">
        <q-card>
            World
        </q-card>
    </div>
</div>

or

julia> htmldiv(@gutter :md [
                card("Hello", sm = 12, lg = 4)
                card("World", sm = 12, md = 8)
              ]) |> prettify |> println
<div class="q-gutter-md">
    <div class="col-sm-12 col-lg-4">
        <q-card>
            Hello
        </q-card>
    </div>
    <div class="col-sm-12 col-md-8">
        <q-card>
            World
        </q-card>
    </div>
</div>

[hhaensel]

Didn't add tests yet. But this is a working PR, finally!

[hhaensel]

Shall we do a squash and merge?