Add more Monero explorers options

[Ulmo0]

For now there's only monero.com explorer by default. Would be nice to add more choices in clearnet and more privacy friendly too like in Tor and i2p.
Some examples Tor and i2p:

http://moneroisfymdhmyrsrjpvugq3w73m3ycnoqckjdedrtflcp56he6jkid.onion/

http://moneroexplorer.i2p/