Copy tables as markdown

[wheelercj]

Depends on #19.

With the setting "when creating markdown of a selection, try to keep the same format", copying a table should result in a markdown table.

For example, copying the selection in this image:

should give this markdown:

| Syntax | Description |
| --- | --- |
| Header | Title |
| Paragraph | Text |

which renders as:

Syntax	Description
Header	Title
Paragraph	Text

It's fine if the resulting markdown has consistent cell widths like this:

| Syntax      | Description |
| ----------- | ----------- |
| Header      | Title       |
| Paragraph   | Text        |