company-modify-line: Handle an empty first line with line-prefix

* Fixes #24 follow-up.
* Cleaner code, less display magic.
company-modify-line: Barf the concat
Set emacs-lisp-docstring-fill-column to 75,

wrap some overly long lines.
company-backends: Tweak the documentation
company-doc-buffer: Use the arg; rename the buffer
Release 0.6.10