Add optional rowspan support for cells marked with '^^'

[pmccloghrylaing]

Closes #15

[redbug312]

Because markdown-it disables es6 linting support on purpose, the arrow functions have to be rewritten in the legacy way. I just pushed consistent-eslintrc branch containing updated lint rules with markdown-it, you may need to rebase on it.
Sorry that I've disabled travis-ci integration by accident. It's now enabled and you could use make test to lint and run tests on local machine.

[pmccloghrylaing]

Branch has been rebased and I've bumped the package version as well.

[redbug312]

Thanks! Please assign consistent-eslintrc as the merged branch. I will merge these branches into master in order.