Add support for alignments in Markdown tables
This PR adds supports for column alignments for GitHub-Flavored Markdown (i.e. the github table format). This has been requested on issue #53 and reuse the same code as the pipe format.
Additionally, this PR allows the elements used to produce the alignments segments to be configured by the way of the begin=, hline=, sep= and end= parameters. This will help support of other formats in the future.
And finally, this PR tweak the github format to introduce a space separation between the pipes separating the cells of the table. This is to conform to the GitHub Flavored Markdown Spec.
This closes #53 and also supersedes #260.
All tests are passing and successful. This PR is ready to be merged upstream.