new regex attribute length

This pull request implements "Add multiline highlight (#100)" by basically adding a new attribute to the linter regex called length. If length is not None, then its passed to the highlight.range function (unmodified) that will higlight the a "length" number of characters from the beginning of the line and if this value is grater than the line length, it will continue marking characters in the following line, effectively creating a highlight block similar to the one shown in the issue #100