Remove requireindex, for browserify support

[deathcap]

Not the most ideal fix but I couldn't find any other feasible solution unfortunately. Background in deathcap/wsmc#19 (comment) but long story short, this change allows using require('mineflayer') in a browserified environment.

Previously I was using require('mineflayer/lib/block') etc. and not accessing the mineflayer index.js at all, but this changed in ca365c3 so I tried to use require('mineflayer').Block instead but then it runs the unbrowserifiable requireindex module. With this PR, index.js can be browserified.

Another option is to include a separate browser.js listing the modules and keep requireindex in the main index.js, but this means it has to be maintained separately. I think making the change to index.js is the simplest solution, although it does mean that this file will have to be updated when new plugins are added. Open to other suggestions but this is the best I could come up with so far (see deathcap/wsmc#19 (comment))

[deathcap]

For what it's worth, I faced a similar problem with voxel-plugins (which also runs under browserify). Various tricks with require() are possible but it is more straightforward and easier to understand in my opinion to make each require explicit (voxel/voxel-plugins#8).

[rom1504]

Yeah, I think it's a good idea to get rid of dynamic requires in general anyway. It's not really a big effort to add a line in index.js for a new plugin.