Feature: theming support for list of subcommand aliases

[janluke]

This PR adds two new styles to HelpTheme which affects a Group subcommands definition list:

• alias: color of a subcommand alias names
• alias_secondary: color of separator (comma) and parenthesis in subcommand alias lists.

If alias_secondary is not provided, alias style is used.

This PR changes the dark theme so that aliases are shown in "yellow" (col1 is bright yellow) and alias_secondary is just normal white.

Closes #151.

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Old, previous iteration

This PR adds three styles to HelpTheme:

• alias: color of alias
• alias_list_sep: color of alias separators (i.e. "," by default)
• alias_list_boundaries: color of eventual characters "wrapping" the list of alias (i.e. parenthesis by default).

The last two might not be specified. In such case:

• col1 will be used for alias_list_sep
• whatever is used for alias_list_sep will be used for boundaries (i.e. alias_list_sep or col1).

Remember you can always use override Command.format_subcommand_aliases to customize how aliases are formatted.

@wabiloo

[wabiloo]

LGTM

[janluke]

I replaced the three styles with just two: alias and alias_secondary. I refactored and added some simple test. I also updated the dark theme to use these new two styles.