Output token to file, fix user agent and panic

[uhthomas]

Also:

• Changed the user agent to hackney/1.17.0 as it's the only magic string I can get to work.
• Fixed a panic caused by deferring the response body in a function closure:

  
  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x744345]

goroutine 1 [running]: main.(auth).login.func1(0xc000233978) /home/thomas/code/github.com/bogosj/tesla/cmd/login/auth.go:120 +0x25 main.(auth).login(0xc000233df8, 0x864b30, 0xc0001aa000, 0xc000120120, 0xc, 0xc00014c090, 0x10, 0x0, 0xc000124060, 0x85f920, ...) /home/thomas/code/github.com/bogosj/tesla/cmd/login/auth.go:156 +0x9ed main.(*auth).Do(0xc000233df8, 0x864b30, 0xc0001aa000, 0xc000120120, 0xc, 0xc00014c090, 0x10, 0x0, 0x0, 0x0, ...) /home/thomas/code/github.com/bogosj/tesla/cmd/login/auth.go:73 +0x12c main.login(0x864b30, 0xc0001aa000, 0x0, 0x0) /home/thomas/code/github.com/bogosj/tesla/cmd/login/main.go:146 +0x5bc main.main() /home/thomas/code/github.com/bogosj/tesla/cmd/login/main.go:182 +0x3d exit status 2

This means golang-ci lint will not be happy. I think it's possible to exclude this pattern?

Fixes #25

[bogosj]

golang-ci doesn't seem to be complaining now about errcheck... odd.