Add an option to set the character for the padding

[zhiburt]

Currently the default character we use is a space character .

The idea is to set this character to a custom character.

let indent = tabled::Indent::new(0, 0, 1, 1).set_padding_char('V', '^', '<', '>');

│vvvvvvvvvvvvvvv│vvvvvvvvvvvvvvv│
│> row 1 col 1 <│> row 1 col 2 <│
│^^^^^^^^^^^^^^^│^^^^^^^^^^^^^^^│

The name of the function is not perfect.
Please if you have a better idea use it.

The changes must be done in Indent structure and in papergrid::Settings.

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inspired by the same logic in https://github.com/vdmeer/asciitable

[zhiburt]

done in #85