Add support for named matching groups.

[NorbertSzydlik]

Express 4.x crashes when using named capturing group in regexp (see expressjs/express#4277 ).
As far as I know, this feature have been added in newer version of path-to-regexp. It seems that it will land in Express 5.
This change aims to add the feature in current version of Express, which still uses path-to-regexp 0.1.x.

[blakeembrey]

Published as 0.1.8 although it could be a breaking change if people on the previous version were relying on it not matching in some way. The names do not change, only the index in the array, and only if they're using this feature of regex.