Add `savecursor` option

Add `savecursor` option

This adds the `savecursor` option which will remember where the cursor
was when the file was closed and put it back when the file is opened
again. The option is off by default so that people aren't confused as to
why the cursor isn't at the start of a file when they open it.

This commit also adds a more general ability to serialize a buffer so
various components can be saved (which could also be useful for persistent
undo).

Fixes #107