CLI improvement: nickel eval and positional file arguments

[yannham]

Is your feature request related to a problem? Please describe.

The default behavior of the Nickel command is to deeply evaluate a program and pretty-print the result. Experience has shown that it's not very intuitive: all other actions correspond to a subcommand, and what's more, evaluation is not necessarily the most used one (it's probably nickel export). So, it doesn't make a lot of sense to have it be the default behavior.

A second detail is to have to use nickel export -f foo.ncl. It's cool that Nickel uses stdin by default, to allow piping with other commands, but most of the time we want to export a specific file: it's the main expected argument of most subcommand. In that case, we could use positional arguments instead and just write nickel export foo.ncl. As a natural extension, we could provide multiple Nickel files, the semantics being to merge them all: nickel export network.ncl firewall.ncl cpu.ncl would be equivalent to nickel export <<< '(import "network.ncl") & (import "firewall.ncl") & (import "cpu.ncl")'.

Describe the solution you'd like

Get rid of the default behavior of nickel: that is, you have to specify a subcommand, and add a subcommand nickel eval which would correspond to the current plain nickel invocation.

Get rid of the argument -f/--file and use positional arguments instead to pass files to evaluate.

[vkleen]

Done in #1632