added preamble example

- fixed preamble section ID so toc link works
- added preamble.png image
- resolves https://github.com/asciidoctor/asciidoctor-documentation/issues/17
new content and examples for column specifier section

- added general table content and examples
- added specifier descriptions and examples
- applies to issue https://github.com/asciidoctor/asciidoctor-documentation/issues/23
Merge pull request #117 from graphitefriction/preamble-example

added preamble example
Merge pull request #118 from graphitefriction/table-attr-additions

new content and examples for column specifier section
expanded table section of user manual

- added content and examples to cell specifier section
- expanded and formatted table summary section
- applies to issue https://github.com/asciidoctor/asciidoctor-documentation/issues/23
Merge pull request #119 from graphitefriction/table-attr-additions

expanded table section of user manual
expanded data format and table format sections

- provided additional data format examples
- added a tracks.csv file so the include:: directive works
- added column width specifier examples
- add section on table width and frame
- added footer section
- rearranged sub-section order
- minor copyedits to text and examples
- resolves issue https://github.com/asciidoctor/asciidoctor-documentation/issues/23
Merge pull request #120 from graphitefriction/table-attr-additions

expanded data format and table format sections