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/mod.rs

@@ -0,0 +1,27 @@
+/*!
+
+This module is responsible for parsing command lines (`Arglist`, an alias for `&[&str]`) into `Command` instances.
+It removes some redundancy from the command line, for example combining the multiple ways to modify a task into a single `Modification` struct.
+
+The module is organized as a nom parser over ArgList, and each struct has a `parse` method to parse such a list.
+
+The exception to this rule is the `args` sub-module, which contains string parsers that are applied to indivdual command-line elements.
+
+All of the structs produced by this module are fully-owned, data-only structs.
+That is, they contain no references, and have no methods to aid in their execution -- that is the `invocation` module's job.
+
+*/
+mod args;
+mod command;
+mod filter;
+mod modification;
+mod report;
+mod subcommand;
+
+pub(crate) use command::Command;
+pub(crate) use filter::Filter;
+pub(crate) use modification::{DescriptionMod, Modification};
+pub(crate) use report::Report;
+pub(crate) use subcommand::Subcommand;
+
+type ArgList<'a> = &'a [&'a str];