Refactor and additional features

* The option `disable="headers"` is deprecated and will be removed in a
future version. Use `disable="CSVColHeaders"` instead.
* The option `headers` is deprecated and will be removed in a future
version. Use `colheaders` instead.
* Added option `enable` that accepts a comma-separated list of features
to explicitly enable.
* Added features `CSVColHeaders` and `CSVRowHeaders` to options
`enable=` and `disable=`.
* Added option `colheaders` to add user-supplied headers to columns.
* Added option `rowheaders` to add user supplied headers to rows.
* Added option `linktosource` to add a link to the source CSV file.
* Added option `tableclass` to add a class attribute to the `<table>`
tag.
* Newlines now preserved in the output by replacing them with `<br>`
tags.
* Fixed a bug where the filename would be missing the leading / if a
URI was given as the CSV source.
* Fixed a bug that missed the opening `<tr>` after the `<thead>` tag.