Implementation of new flex backend

[joto]

This PR introduces a new flex backend, see #1036 for the ideas behind this.

If you want to try this: Compile as usual, Lua support must be enabled. Then run osm2pgsql with -O flex and -S some-config.lua. Docs in docs/flex.md. Look into flex-config directory for some example config/style files with inline documentation. Start with simple.lua.

This PR more or less resolves the following issues: #109, #153, #230, #277, #328, #573, #677, #734, #901

In addition this PR impacts the following issues: #107, #328, #768

[lonvia]

I've looked through the code and this looks good for a first experimental implementation. There might be room for consolidation with the existing code but I don't think it's worth doing that before the interface of the output is stable.

I would suggest to mark this as experimental and merge this in its current state. This makes it easier for people to test the new output and also means that we can have smaller PRs with further improvements. @pnorman what do you think?

@joto can you please add the flex output to the existing READMEs with a big fat warning that this is experimental and the API might change without notice.

[pnorman]

I would suggest to mark this as experimental and merge this in its current state. This makes it easier for people to test the new output and also means that we can have smaller PRs with further improvements. @pnorman what do you think?

I'm okay with this.

[jeisenbe]

Thank you, this looks very useful. Will this be part of an official release soon?

[lonvia]

It will be part of the next release but I cannot give an ETA on that. We need some more testing (and more tests). So, the more people try out the master branch or volunteer tests, the sooner the release. ;)