Header tags off-by-one error, and class-prefix issues

[JohnPostlethwait]

Ruby version: ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]
Gem version: 2.5.1

Make a .lsg file with:

# This should be a H1
## This should be a H2
### This should be a H3
#### This should be a H4
##### This should be a H5
###### This should be a H6

Compile it via the command line.

The outputted HTML is this:

<h2 class="lsg-headline" id="this-should-be-a-h1"><a class="lsg-anchor" href="#this-should-be-a-h1"></a>This should be a H1</h2>
<h3 class="lsg-sub-headline" id="this-should-be-a-h2"><a class="lsg-anchor" href="#this-should-be-a-h2"></a>This should be a H2</h3>
<h4 class="lsg-sub-sub-headline" id="this-should-be-a-h3"><a class="lsg-anchor" href="#this-should-be-a-h3"></a>This should be a H3</h4>
<h5 class="lsg-" id="this-should-be-a-h4"><a class="lsg-anchor" href="#this-should-be-a-h4"></a>This should be a H4</h5>
<h6 class="lsg-" id="this-should-be-a-h5"><a class="lsg-anchor" href="#this-should-be-a-h5"></a>This should be a H5</h6>
<h7 class="lsg-" id="this-should-be-a-h6"><a class="lsg-anchor" href="#this-should-be-a-h6"></a>This should be a H6</h7>

As you can see, the class names are identical for H3's through H6's (which are off-by-one and render as H(n+1)).

All Header levels should get unique lsg- selectors, and the Markdown should not render as Hn+1.