added reachability / velocity plugins

[trafficonese]

I included 2 new plugins:

• leaflet.reachability
• leaflet-velocity

This should close #125, #175, #178

Unfortunately I dont know exactly what the webpack.config.js is doing or how I could include the plugins there, since the plugins are not available for npm.

I could also not run npm install as I got this error:

> [email protected] postinstall C:\...\leaflet.extras\node_modules\leaflet-pulse-icon
> gobble build -f dist

gobble[19552]: c:\ws\src\node_contextify.cc:635: Assertion `args[1]->IsString()' failed.
 1: 00007FF6E775C6AA v8::internal::GCIdleTimeHandler::GCIdleTimeHandler+4506
 2: 00007FF6E7737416 node::MakeCallback+4534
 3: 00007FF6E77374CF node::MakeCallback+4719
 4: 00007FF6E770B7C0 node::DecodeWrite+13120
 5: 00007FF6E7C64382 std::vector<v8::internal::compiler::MoveOperands * __ptr64,v8::internal::ZoneAllocator<v8::internal::compiler::MoveOperands * __ptr64> >::_Umove+79442
 6: 00007FF6E7C654E9 std::vector<v8::internal::compiler::MoveOperands * __ptr64,v8::internal::ZoneAllocator<v8::internal::compiler::MoveOperands * __ptr64> >::_Umove+83897
 7: 00007FF6E7C6482C std::vector<v8::internal::compiler::MoveOperands * __ptr64,v8::internal::ZoneAllocator<v8::internal::compiler::MoveOperands * __ptr64> >::_Umove+80636
 8: 00007FF6E7C6474B std::vector<v8::internal::compiler::MoveOperands * __ptr64,v8::internal::ZoneAllocator<v8::internal::compiler::MoveOperands * __ptr64> >::_Umove+80411
 9: 000001DBE625C5C1
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: [email protected] (node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for [email protected]: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})

npm ERR! code ELIFECYCLE
npm ERR! errno 134
npm ERR! [email protected] postinstall: `gobble build -f dist`
npm ERR! Exit status 134
npm ERR!
npm ERR! Failed at the [email protected] postinstall script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

[trafficonese]

I wanted to create a second PR for the second commit, but I am not experienced enough with git..
I hope thats ok anyway?

The second commit should close #148, #156, #165