Need to support extension rows

[benosteen]

"Extensions: structured comment lines

Comment lines that begin with a token of the form '#%symbol' are special structured comment lines that usually indicate specific optional functionality that extends the core Checkm specification. Matching against a symbol is case-insensitive (e.g., #%foo is equivalent to #%FOO). The rest of a structured comment line is tokenized in the same way as non-comment lines. The structured comment symbols that follow are currently reserved."