Feature request: column separators

[njourdane]

It could be practical to have column separators.

Syntax proposal:

print(tabulate(table, headers=[["item", "qty", "foo"], ["bar", "baz"]]))

Could give, using fancy_outline format:

╒════════╤═══════╤═════╦═════╤═════╕
│ item   │ qty   │ foo ║ bar │ baz │
╞════════╪═══════╪═════╬═════╪═════╡
│ spam   │ 42    │ a   ║ d   │ g   │
│ eggs   │ 451   │ b   ║ e   │ h   │
│ bacon  │ 0     │ c   ║ f   │ i   │
╘════════╧═══════╧═════╩═════╧═════╛

Related to #275