Support flexible SelectFocus text width

[catc]

Allow for icons.SelectFocus.Text to have a length > 1 and still align options correctly.

For example:

func setIcons(icons *survey.IconSet) {
	icons.SelectFocus.Text = "🌿"
}

results in misaligned select options:

This might be a little more difficult for emojis (in my case, 🌿 is 4 bytes but has a width of 2 in the terminal) - https://github.com/rivo/uniseg might come in useful.

[AlecAivazis]

Thanks for reporting this @catc!