Fix stackoverflow when beautifying table with unicode

`nchar()` does count the number of chars. `stringr::str_pad`, however,
builds upon the string length. To account for that, all `nchar()` based
width metrics were moved to `stringi::stri_width`.

A test was added to monitor unicode beautification.

Closes #14

Another fix of comments, introduction of testthat

This patch fixes futher issues related to comment formatting. Closes #10

Also, this patch introduces proper testing using testthat.

Fix single-word backtick elements in comments

Add functionality to beautify comments

A new addin "Beautify Comment" allows for the alignment of multi-line
comments to respect the 80 column margin.

Closes #6