Add support for required via tag (if given inside OSM data)

[ialokim]

This PR adds support for the required via tag if it is set in the OSM data.

After merging in #126, this PR should be rebased and we should change the verification check introduced there by adding the via tag.

Fixes #101.

[xamanu]

I was first hesitating about the via tag, as it doesn't seem to be an official tag in OpenStreetMap. However it is mentioned in the wiki, part of the approved proposal, and it is used quite extensively. So, all good, to include it into the osm2gtfs core. It looks good so far. Will wait until #126 got in until I test it thoroughly.

[ialokim]

I've only rebased and made pep8 happy 😆

[grote]

Thanks @ialokim for all these bug fixes! :)

[grote]

It was already like this, but breaking open the accessor like this looks like bad style. Can we keep trip["via"] together without line breaks?

[ialokim]

I've changed it accordingly, it's now like in line 87.

[ialokim]

Before merging this, we should check all the creators' GTFSes, because of the new required information in the schedule file. I had to fix Esteli's schedule accordingly.

[xamanu]

I guess the tests (#133, #135, #136 and #137) will be very useful for this.

[grote]

Thanks @ialokim!