Preserve cursor position when sending a line or paragraph

When you send a line or a paragraph the cursor moves to the beginning, which is OK, most of the time. But sometimes you may want to test something and not lose track of the current line (especially if it's a function definition spanning multiple lines, or a long line).

I added `g:slime_preserve_curpos` to define if it should preserve the cursor position, because I'm not sure if this is something that you'd want as a default. It makes the whole "sending a paragraph" too silent.

But I'm using vim-slime for livecoding with [Tidal](http://yaxu.org/tidal/) and, in that context, this is a really good feature :)

What do you think?