main.getValidCwd() breaks on cygwin

[kylewolfe]

https://github.com/justjanne/powerline-go/blob/master/main.go#L95-L101

I receive this warning in almost every directory except ~: [powerline-go]Your current directory is invalid. Lowest valid directory:

I suggest the following possible fixes:

• Option to disable valid directory checking (is this overhead even needed?)
• Check if $OSTYPE == cygwin
  • Route through cygpath -m $pwd
  • Turn off valid directory check
• Provide a global CLI option for the user to provide the cwd or pwd that will work. Eg: PS1="$($GOPATH/bin/powerline-go -modules="host,user,git,kube,cwd" -pwd $(cygpath -m `pwd`) -error $?)"

I think the last option seems the cleanest, least invasive, and quickest to implement.

Related to #12.

[kylewolfe]

PR #124 has been submitted.

[justjanne]

Could you try this with the changed implementation now again?