Eicar update

This adds EICAR payload encoding, EXE::EICAR, and MSI::EICAR options to the EXE generator.

## Validation

**EICAR payload encoding**

- [x] Run the following command: `./msfvenom -p osx/x64/shell_bind_tcp -f raw -e generic/eicar`
- [x] See the expected EICAR string (with no linefeed at the end)

**EXE generator when EXE::EICAR is FALSE**

- [x] `use exploit/windows/smb/psexec`
- [x] Configure the psexec per normal -- set RHOST, SMBUser, and SMBPass
- [x] Fire up ngrep and search for EICAR: `ngrep -d vmnet8 EICAR port 445`
- [x] run `exploit`
- [x] See the lack of EICAR, like so:

````
root@mazikeen:~/git/rapid7/metasploit-framework
# ngrep -d vmnet8 EICAR port 445
interface: vmnet8 (192.168.145.0/255.255.255.0)
filter: (ip or ip6) and ( port 445 )
match: EICAR
###############################################################################################################################
````
- [x] Note that a shell session is created.

**EXE generator when EXE::EICAR is TRUE**

- [x] Set `EXE::EICAR true`
- [x] See the presence of EICAR:

````
root@mazikeen:~/git/rapid7/metasploit-framework
# ngrep -d vmnet8 EICAR port 445
interface: vmnet8 (192.168.145.0/255.255.255.0)
filter: (ip or ip6) and ( port 445 )
match: EICAR
#################
T 192.168.145.1:55936 -> 192.168.145.60:445 [AP]
  .....SMB/......(..l....fX.....j..PM*.................D...D.?.....D.X5O!P%@A
  P[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*               
####################################################
````

- [x] Note that no shell session is created.