Function: expand_keywords

--Usage--
> expand_keywords(some_tagged_string, default_expand: {...}, combine: {...})

Find [[type:keyword]]s and generate their reminder text.

The string is searched for keywords, when one is found its reminder text is generated.
This is done by calling @keyword.reminder(param1:.., param2:..., ...)@.
Where the @param@s are the matching keyword parameters.

The it is determined whether the reminder text should be shown.
This is either because the user explicitly selected it, or when @default_expand(mode: keyword.mode)@ returns true.
Whether reminder text is shown is stored in a @<kw->@ tag around the keyword.
! Tag		Reminder text		Changed by user
| @"<kw-a>"@	hidden			no
| @"<kw-A>"@	shown			no
| @"<kw-0>"@	hidden			yes
| @"<kw-1>"@	shown			yes

If the reminder text is shown the function @combine@ is called which should combine the reminder text with (in the variable @reminder@) with the keyword and its parameters (in the variable @keyword@).

For example, in the case of magic:
> expand_keywords(
>   default_expand: { set.automatic_reminder_text }
>   combine:        { "{keyword}<atom-reminder-{mode}> ({reminder})</atom-reminder-{mode}>" }
> )
is used. This shows reminder text by default based on a [[type:set]] option, and it combined the keyword as @"keyword (reminder)"@.

--Parameters--
! Parameter		Type				Description
| @input@		[[type:tagged string]]		String to expand keywords in.
| @default_expand@	[[type:function]]		Should reminder text be shown by default?
| @combine@		[[type:function]]		How to combine keywords with the reminder text?

--Examples--
Assuming a keyword @"mse"@ exists with reminder text @"Magic Set Editor"@ exists.
> expand_keywords(default_expand: {true}, combine: { keyword + " = " + reminder },  "mse is cool")
>    == "<kw-A>mse = Magic Set Editor is cool</kw-A>"
>
> f := expand_keywords@(default_expand: {true}, combine: { keyword+"="+reminder })
> f("mse is cool") == "<kw-A>mse = Magic Set Editor is cool</kw-A>"