1. UsageText shows the command usage rule. Fixed/added where needed.
2. Description shows the command description, huh. It is shown right after
UsageText, so there's no need to repeat the command usage rule. If
Description contains Example, then it should be printed on a new line.
Usage message is shown on common --help command, thus it should be meaningful
and short. If user needs more detailed command description, then he can use
command-specific help.
As a result, current VM help looks pretty simple:
```
NEO-GO-VM > help
NAME:
VM CLI - Official VM CLI for Neo-Go
USAGE:
[global options] command [command options] [arguments...]
VERSION:
0.99.5-pre-15-g5463ec41
COMMANDS:
exit Exit the VM prompt
ip Show current instruction
break Place a breakpoint
jump Jump to the specified instruction (absolute IP value)
estack Show evaluation stack contents
istack Show invocation stack contents
sslot Show static slot contents
lslot Show local slot contents
aslot Show arguments slot contents
loadnef Load a NEF-consistent script into the VM optionally attaching to it provided signers with scopes
loadbase64 Load a base64-encoded script string into the VM optionally attaching to it provided signers with scopes
loadhex Load a hex-encoded script string into the VM optionally attaching to it provided signers with scopes
loadgo Compile and load a Go file with the manifest into the VM optionally attaching to it provided signers with scopes
loadtx Load transaction into the VM from chain or from parameter context file
loaddeployed Load deployed contract into the VM from chain optionally attaching to it provided signers with scopes
reset Unload compiled script from the VM and reset context to proper (possibly, historic) state
parse Parse provided argument and convert it into other possible formats
run Execute the current loaded script
cont Continue execution of the current loaded script
step Step (n) instruction in the program
stepinto Stepinto instruction to take in the debugger
stepout Stepout instruction to take in the debugger
stepover Stepover instruction to take in the debugger
ops Dump opcodes of the current loaded program
events Dump events emitted by the current loaded program
env Dump state of the chain that is used for VM CLI invocations (use -v for verbose node configuration)
storage Dump storage of the contract with the specified hash, address or ID as is at the current stage of script invocation
changes Dump storage changes as is at the current stage of loaded script invocation
help, h Shows a list of commands or help for one command
GLOBAL OPTIONS:
--help, -h show help
--version, -v print the version
```
Share parameters parsing code between 'contract invokefunction' and
'vm run' commands. It allows VM CLI to parse more complicated parameter
types including arrays and file-backed bytestrings.
