Allow custom top in grammars

[sertonix]

While most grammars will want the wrapper around the token it may be bad in some cases.
It would be nice if a grammar defines top it would be used instead of the default top.

flare/language.go

Lines 111 to 120 in f3146ac

	top := p.Or(
	token,
	p.Concat(
	p.Any(1),
	p.Star(p.Concat(
	p.Not(token),
	p.Any(1),
	)),
	),
	)

In markdown for example headings can be at the start of the line.
This can be achived by splitting all lines (top <- (!\n line) \n?).
This would be possible with the token but it would be more of a hack and less efficient.

To solve the problem in markdown a character for start of the line and end of the line just like in regex would also be possible. I would say it would even say it would be the least hacky way. The problem is that it makes the code more complex and it would maybe require some backtracking.

Another note: When defining a custom top it would bypass the memo option. No idea how bad/good that is.