feat(tables): add markdown and ascii border style for tables

[andreynering]

Closes #424

Markdown style:

|----------|--------------|-----------|
| LANGUAGE |    FORMAL    | INFORMAL  |
|----------|--------------|-----------|
| Chinese  | Nǐn hǎo      | Nǐ hǎo    |
| French   | Bonjour      | Salut     |
| Russian  | Zdravstvuyte | Privet    |
| Spanish  | Hola         | ¿Qué tal? |
|----------|--------------|-----------|

ASCII style:

+----------+--------------+-----------+
| LANGUAGE |    FORMAL    | INFORMAL  |
+----------+--------------+-----------+
| Chinese  | Nǐn hǎo      | Nǐ hǎo    |
| French   | Bonjour      | Salut     |
| Russian  | Zdravstvuyte | Privet    |
| Spanish  | Hola         | ¿Qué tal? |
+----------+--------------+-----------+

[meowgorithm]

Nice! It looks like Markdown wants the bottom and top sections of the Markdown one gone, i.e.

| LANGUAGE |    FORMAL    | INFORMAL  |
|----------|--------------|-----------|
| Chinese  | Nǐn hǎo      | Nǐ hǎo    |
| French   | Bonjour      | Salut     |
| Russian  | Zdravstvuyte | Privet    |
| Spanish  | Hola         | ¿Qué tal? |

Rendered that looks like:

LANGUAGE	FORMAL	INFORMAL
Chinese	Nǐn hǎo	Nǐ hǎo
French	Bonjour	Salut
Russian	Zdravstvuyte	Privet
Spanish	Hola	¿Qué tal?

[andreynering]

Hey @meowgorithm. Thanks for noticing that.

Unfortunately, it seems that it is not possible to address that by just changing that configuration, because:

1. The char for the divider between headers and data is shared with the top and down char. Changing that makes the rendering of the divider broken.
2. Even at top and bottom we'd have a bug of having empty lines being printed, instead of ommited.

So, to me, the right solution is to actually disable top and bottom borders with .BorderTop(false).BorderBottom(false). I added both as examples to the README to help the user. What do you think?

[meowgorithm]

@andreynering Okay yeah, that makes sense. So let's see if docs will suffice for now. If not, we can cautiously consider a shorthand for markdown rendering later.

On that note I'd add the note about .BorderTop(false).BorderBottom(false) to the markdown border GoDocs as well, then this is good to go.