Allow empty keywords.

src/parser/keyword.rs

@@ -5,6 +5,7 @@ use nom::character::complete::line_ending;
 use nom::character::complete::space0;
 use nom::character::complete::space1;
 use nom::combinator::eof;
+use nom::combinator::opt;
 use nom::combinator::recognize;
 use nom::sequence::tuple;
 
@@ -22,8 +23,7 @@ pub fn keyword<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str,
         tag("#+"),
         is_not(" \t\r\n:"),
         tag(":"),
-        space1,
-        is_not("\r\n"),
+        alt((recognize(tuple((space1, is_not("\r\n")))), space0)),
         alt((line_ending, eof)),
     )))(input)?;
     Ok((remaining, Keyword { source: rule }))