Implement command-line interface using structopt

This patch adds the structopt dependency and implements a simple
command-line interface for rusty-man.

Cargo.lock

@@ -2,7 +2,173 @@
 # It is not intended for manual editing.
 # SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
 # SPDX-License-Identifier: CC0-1.0
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Cargo.toml

@@ -13,3 +13,8 @@ readme = "README.md"
 license = "MIT"
 
 [dependencies]
+anyhow = "1.0.31"
+
+[dependencies.structopt]
+version = "0.3.15"
+default-features = false

src/main.rs

@@ -1,6 +1,24 @@
 // SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
 // SPDX-License-Identifier: MIT
 
-fn main() {
+use structopt::StructOpt;
-    println!("Hello, world!");
+
+/// Command-line interface for rustdoc documentation
+#[derive(Debug, StructOpt)]
+struct Opt {
+    /// The keyword to open the documentation for, e. g. `rand_core::RngCore`
+    keyword: String,
+
+    /// The sources to check for documentation generated by rustdoc
+    ///
+    /// Typically, this is the path of a directory containing the documentation for one or more
+    /// crates in subdirectories.
+    #[structopt(name = "source", short, long, number_of_values = 1)]
+    source_paths: Vec<String>,
+}
+
+fn main() -> anyhow::Result<()> {
+    let opt = Opt::from_args();
+    println!("Documentation for {}", &opt.keyword);
+    Ok(())
 }