Currently cli silently (aside from IntSlice and Int64Slice which oddly
printed directly to the error stream) ignores failures that occur when
parsing environment variables for their value for flags that define
environment variables. Instead, we should propogate up the error to the
user.
This is accomplished in a backwards compatible manner by adding a new,
internal, interface which defines an applyWithError function that all
flags here define. In v2, when we can modify the interface, we can drop
this secondary interface and modify `Apply` to return an error.
When assigning values to flags (also when interogatting via
`context.(Global)IsSet`.
For boolean flags, consider empty as `false`.
Using `syscall.Getenv` rather than `os.LookupEnv` in order to support
older Golang versions.
by defining `cli.DefaultFlagStringFunc` with a default value that uses
`withEnvHint`, conditionally running a given flag's `FormatValueHelp` if
present.
Closes#257
Feels like it may have been copied from StringSliceFlag or something,
but update the output to be more consistent with other single value
flags.
Also added comments to the String and Apply functions.
