Add support for custom stringLength calculator callback function

The text-table module uses JavaScripts own `String(s).length` method to return the length of a string. This is incorrect when using ANSI color coding or UTF8 characters - as there is text that needs echoing to the screen but is not actually visible to the user.

This pull adds support for providing an alternate stringLength callback function which can correctly calculate the cell widths during render.

I've also provided a test script to verify that ANSI encoding is correctly applied on the CLI.

You may wish to update the package version (so the NPM repo is updated) and *possibly* add `cli-color` to your devDependencies list.