Add setting for the GOROOT environment variable

[cixtor]

Users might choose to set a different value for the GOROOT environment variable than the standard, this confuses some of the linters like the one that checks for the imported packages because it is not able to find them in the assumed GOROOT from the system. Users with a custom dev env should be able to set their own variables from the settings, example:

"gometalinter": {
  "disable": false,
  "args": [],
  "excludes": [],
  "goroot": "/opt/golang/build",
  "gopath": "/opt/golang/gopath"
}

[alecthomas]

Seems like a good idea, thanks!