Alias categories

Pull request system aliases should use the CakeAliasConstants.MainCakeAliasCategory and CakeAliasConstants.PullRequestSystemCakeAliasCategory constants for defining their category:

[CakeAliasCategory(CakeAliasConstants.MainCakeAliasCategory)]
public static class MyPullRequestSystemAliases
{
    [CakeMethodAlias]
    [CakeAliasCategory(CakeAliasConstants.PullRequestSystemCakeAliasCategory)]
    public static ICodeAnalysisProvider MyPullRequestSystem(
        this ICakeContext context)
    {
    }
}