Optional directory for local help files

[zandermax]

The generated help files should be able to be located wherever the user wants.

Example command:

sfdx.update({
    helpdir: '/home/sfdx/docs'
})

The directory should be made (possibly with confirmation) if not present.

[zandermax]

Looks like the CLI no longer has the help commands? However, they are still in the official CLI documentation for the current version. Thus, I'll leave this open for the time being.