Datapasta

* Added new addin 'Paste as vector (vertical)' to provide nicer formatting for long lists.
* All addins now guess data types and format correctly in the source editor.
* Empty rows in tables and empty cells in lists are formatted as NA's when pasting instead of being ignored.
* Added vignette, automated tests etc in prep for CRAN submission.

* Added graceful error handling on failed parse of text on clipboard to table.
* `tribble_paste()` and `vector_paste()` now pastes NA's as unquoted, so R will parse as proper NA.
* `tribble_paste()` can parse an paste table text copied from raw delimited file e.g. csv, tsv, pipe delimited, semi-colon delimited.
* `vector_paste()` uses a space between elements.

* Added a `NEWS.md` file to track changes to the package.
* Fixed the handling of NAs in tab delimited files which resulted in phantom NA columns sometimes appearing with `tribble_paste()`