Add per-line anchors

[fantasai]

Please add per-line anchors so that it is possible to link into the middle of a long email. It would be nice to add this, something like

<span id="L23">  and therefore in the second step of the algorithm</span>

[jkbzh]

No can do until issue #51 is resolved. Otherwise all those link ids would change when two f=f lines are combined into a single one.