Stick a <div> inside the <body>

The point is to fix #16.

Broadly, the correct solution is "stick a `<div>` inside the body with the foreground/background from the `Normal` highlight group".

How to find out those colors?  Well, `2html.vim` has a lot of logic to figure out what they are; it makes sense to just re-use that logic if we can.  If `g:html_use_css == 0` (which we force to be true), then those colours occur in the `<body>` attribute.  So we can solve this problem by just copying the `<body>` to a `<div>` and keeping the same attributes, right?

Well... almost.  It turns out, Gmail disregards the deprecated `bgcolor=` and `text=` attributes.  So we have to do a _little_ more legwork to turn them into a `style` attribute.  But overall, this isn't too bad.
In vimscript, I prefer to use functions and commands which don't affect user stuff in the first place, rather than saving and restoring user-visible stuff.  Would you like me to add a comment to the function to explain why I'm not just using `:s//`?

I think the name is mostly clear enough so that the calling code is readable, though I guess it would be better as `ReplaceOnCurrentLine` instead of `ReplaceCurrentLine`.  I'll make that change now.