Add support for markdown column alignment

[WillAbides]

resolves #173

This adds support for column alignment in markdown tables so that the rendered markdown table will have the same column alignment as the text version.

An example from a test:

|   DATE   |       DESCRIPTION        | CV2  | AMOUNT |
|----------|:-------------------------|-----:|:------:|
| 1/1/2014 | Domain name              | 2233 | $10.98 |
| 1/1/2014 | January Hosting          | 2233 | $54.95 |
| 1/4/2014 | February Hosting         | 2233 | $51.00 |
| 1/4/2014 | February Extra Bandwidth | 2233 | $30.00 |

renders as:

DATE	DESCRIPTION	CV2	AMOUNT
1/1/2014	Domain name	2233	$10.98
1/1/2014	January Hosting	2233	$54.95
1/4/2014	February Hosting	2233	$51.00
1/4/2014	February Extra Bandwidth	2233	$30.00

[olekukonko]

@mattn what do you think?

[mfridman]

This would be extra fantastic. Especially as more and more apps add proper markdown support.

Anything the community can do to help out? cc @mattn (👋 long time)

[jippi]

Would love to see this land. @olekukonko is the code/approach fine? if yes, I'm happy to do the work to revive this PR so its mergeable :)

[olekukonko]

@jippi will review this in detail and revert ... however this may be highly dependent on the introduction of theme

[jippi]

@olekukonko thank you, I appreciate it :) Please let me know if I can support it in any way as well

[WillAbides]

I noticed the activity here and merged master into the PR branch.

[jippi]

Thank you @WillAbides ! :)

@olekukonko How does it look from your point of view? Does it need any further polishing / tweaks?