sshtunnel: parse ProxyJump

[sweenu]

Closes #244

I stole this snipped of code from netmiko. (MIT license)

Until paramiko/paramiko#1951 gets merged, token parsing won't work but I think we can still merge. It's better than nothing.

[coveralls]

Coverage decreased (-0.2%) to 91.22% when pulling d93bd5f on sweenu:master into 58a46ef on pahaz:master.

[sweenu]

The errors in the pipeline seem unrelated to my changes

[notxvilka]

@pahaz could you please take a look? It's a pity that sshtunnel doesn't support ProxyJump that is a common these days in corporate settings for using jumpservers.