Add user defined buffer number style.

[Dephilia]

This PR add the options to change the style of buffer number to make it looks better. bufnr_style: table, the mapping target. For example, buffer number 21 will map to ²¹ using the configuration.

bufnr_style = { '⁰', '¹', '²', '³', '⁴', '⁵', '⁶', '⁷', '⁸', '⁹' }

bufnr_direction: boolen, set true to put buffer number to left.

[kdheepak]

Thanks for the PR! That is a neat feature!