company--column: Account for line-prefix even at bol

Again, for org-indent-mode.
* company-untabify: Rename to `company-plainify'.
* company-modify-line: Move `line-prefix' logic to `company-plainify'.

Fix #24, take four
company-files-complete: Filter candidates by prefix
Change default `company-begin-commands' value to be more conservative
Bump the version

Oops, forgot to do that the last time