Fix plugin commands

[shargon]

Close #556

[nicolegys]

key.ToLower cannot get the value

[shargon]

@nicolegys please take a look again

[nicolegys]

Thank you @shargon . Now it works well.
Do we need to display all the command help information including the plugins when we call help?
Now it looks like this (StatesDumper and PerformanceMonitor are installed):
a bit messy
Or maybe we need to put all the plugin comands in Category Plugin Commands?

[shargon]

Both options are good to me, what do you prefer? it's enough with change the category of the plugin command.

[nicolegys]

I prefer to have plugin commands in the Plugin Commands catagory, to make the help information more clearly. And whether the plugins are installed or not, they will not affect the categories of neo-cli itself.

Base Commands:
...
Blockchain Commands:
...
Consensus Commands:
...
Contract Commands:
...
Network Commands:
...
Node Commands:
...
Plugin Commands:
...
Wallet Commands:
...

But maybe the plugin commands need to have their own catagories，like this

neo> help PerformanceMonitor
Block Commands:
        block avgtime [blockCount=1000]
        block sync
        block time <blockIndexOrHash>
        block timesincelast
Check Commands:
        check cpu
        check memory
        check threads
Consensus Commands:
        commit time
        confirmation time
        payload time
Network Commands:
        connected
        ping [ipaddress=null]
        rpc time <url>
Transaction Commands:
        tx avgsize [txCount=1000]
        tx size <transactionHash>

I'm not sure if changing all of their categories to Plugin Commands will make the help information more friendly. Or maybe we need subcategories? What do you think?