Switch to a command-line API closer to TaskWarrior

* Use a parser (rather than clap) to process the command line
* Outline some generic support for filtering, reporting, modifying, etc.
* Break argument parsing strictly from invocation, to allow independent testing

cli/src/argparse/command.rs

@@ -0,0 +1,62 @@
+use super::args::*;
+use super::{ArgList, Subcommand};
+use failure::{format_err, Fallible};
+use nom::{combinator::*, sequence::*, Err, IResult};
+
+/// A command is the overall command that the CLI should execute.
+///
+/// It consists of some information common to all commands and a `Subcommand` identifying the
+/// particular kind of behavior desired.
+#[derive(Debug, PartialEq)]
+pub(crate) struct Command {
+    pub(crate) command_name: String,
+    pub(crate) subcommand: Subcommand,
+}
+
+impl Command {
+    pub(super) fn parse(input: ArgList) -> IResult<ArgList, Command> {
+        fn to_command(input: (&str, Subcommand)) -> Result<Command, ()> {
+            let command = Command {
+                command_name: input.0.to_owned(),
+                subcommand: input.1,
+            };
+            Ok(command)
+        }
+        map_res(
+            all_consuming(tuple((arg_matching(any), Subcommand::parse))),
+            to_command,
+        )(input)
+    }
+
+    /// Parse a command from the given list of strings.
+    pub fn from_argv(argv: &[&str]) -> Fallible<Command> {
+        match Command::parse(argv) {
+            Ok((&[], cmd)) => Ok(cmd),
+            Ok((trailing, _)) => Err(format_err!(
+                "command line has trailing arguments: {:?}",
+                trailing
+            )),
+            Err(Err::Incomplete(_)) => unreachable!(),
+            Err(Err::Error(e)) => Err(format_err!("command line not recognized: {:?}", e)),
+            Err(Err::Failure(e)) => Err(format_err!("command line not recognized: {:?}", e)),
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    // NOTE: most testing of specific subcommands is handled in `subcommand.rs`.
+
+    #[test]
+    fn test_version() {
+        assert_eq!(
+            Command::from_argv(argv!["task", "version"]).unwrap(),
+            Command {
+                subcommand: Subcommand::Version,
+                command_name: "task".to_owned(),
+            }
+        );
+    }
+}