Correctly highlight placeholders in Gherkin syntax

[jamis]

While placeholders (variables surrounded by angle brackets, like <this>) are mostly highlighted correctly, there are two cases that are not:

1. When the angle bracket is immediately preceded by text, like something (<foo>), and
2. When the angle bracket is the first non-whitespace text following a step keyword, like When <foo>.

This PR addresses these two cases. Some things to note about this PR.

1. The Text rule in the step state now stops as soon as it encounters either whitespace, OR a left angle bracket. This fixes case number 1.
2. Rouge::Lexers::Gherkin.step_regex has been modified so that it does not added a \b to the end of the regex, if the pattern ends with a whitespace character. In fact, I wonder if there is a disconnect between the step_regex function and the keyword configs, since the comment mentions patterns that end with a <, but there are no patterns in the config that do so. This change, though, fixes case number 2.

[pyrmont]

@jamis Two apologies:

1. I'm sorry it's taken so long to get around to this PR. Rouge had been lying fallow for a while and while I came onto the project in May, I've been working my way through the PR backlog in reverse chronological order. I'm afraid that meant it took a while to get to this one.

2. I'm sorry for force pushing the changes onto your branch. Since you submitted the PR, we've moved to using regex literals that avoid the ambiguity warning Ruby will generate when run with verbose warnings. This meant changing the form of expressions from rule /.../ to rule %r/.../ in a couple of cases. Since I was already making some changes, I also made two other small adjustments.

We're on a two-week cadence for Rouge updates as we clear things up. It's been so long it may not be relevant to you any more but this fix will land in version 3.7.0 of the gem next Tuesday. Thanks for taking the time to submit the fix. Although very belated, it is nevertheless very much appreciated!

[ghost]

Thanks for the follow-up! I appreciate your hard work, and your continued efforts on Rouge.