Replace -convey-json flag with env variable…

[powerman]

…and stop parsing source for goconveyDSLImport, please: https://github.com/smartystreets/goconvey/blob/master/web/server/system/shell.go#L171

It's very inconvenient requirement - add import _ "github.com/smartystreets/goconvey/convey" (for no reason) in each package which wanna get nice diff in goconvey's web UI for usual, non-Convey() tests (supposing this package implements goconvey-compatible JSON output).

Env var is much more simple and clean way to switch on feature which may not exists in executed binary, there is no reason to use flags for everything (especially at cost of needs to parse imports from source).