Support custom colors for drawbox characters

[joelim-work]

Fixes #902

Add a new borderfmt option to allow setting the pane borders to a custom color.

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In addition a new utility function parseEscapeSequence is added, which basically converts a string to a tcell.Style object. I think this function is better than the existing optionToFmtstr function for processing some of the other formatting options like numberfmt and tagfmt.

For example, it is now possible to write this:

win.print(screen, 0, i, parseEscapeSequence(gOpts.numberfmt), ln)

Instead of this:

win.print(screen, 0, i, tcell.StyleDefault, fmt.Sprintf(optionToFmtstr(gOpts.numberfmt), ln))

[gokcehan]

@joelim-work Looks good, thanks for the patch.

[joelim-work]

@asklow Have a look at the borderfmt option in the documentation.