FEATURE: Add a subcommand to describe the ucg environment variables.

Also update documentation with help output.

docsite/site/content/getting-started/_index.md

@@ -77,20 +77,20 @@ Ucg has builtin help on the command line.
 
 ```
 $> ucg help
-ucg 0.2.2
-Jeremy Wall <jeremy@marzhillstudios.com>
 Universal Configuration Grammar compiler.
 
 USAGE:
-    ucg [SUBCOMMAND]
+    ucg [FLAGS] [SUBCOMMAND]
 
 FLAGS:
-    -h, --help       Prints help information
-    -V, --version    Prints version information
+    -h, --help         Prints help information
+        --no-strict    Turn off strict checking.
+    -V, --version      Prints version information
 
 SUBCOMMANDS:
     build         Build a list of ucg files.
     converters    list the available converters
+    env           Describe the environment variables ucg uses.
     help          Prints this message or the help of the given subcommand(s)
     inspect       Inspect a specific symbol in a ucg file.
     test          Check a list of ucg files for errors and run test assertions.

src/main.rs

@@ -55,6 +55,9 @@ fn do_flags<'a, 'b>() -> clap::App<'a, 'b> {
             (@subcommand converters =>
              (about: "list the available converters")
             )
+            (@subcommand env =>
+             (about: "Describe the environment variables ucg uses.")
+            )
     )
 }
 
@@ -377,6 +380,21 @@ fn converters_command(registry: &ConverterRegistry) {
     }
 }
 
+fn env_help() {
+    println!("Universal Configuration Grammar compiler.");
+    println!("");
+    println!("ENVIRONMENT VARIABLES:");
+    println!("");
+    println!(
+        "
+    UCG_IMPORT_PATH=\"{}\"
+      A list of paths to search for imports from. Uses the same syntax
+      as your platforms $PATH environment variable.
+",
+        std::env::var("UCG_IMPORT_PATH").unwrap_or(String::new())
+    );
+}
+
 fn main() {
     let mut app = do_flags();
     let app_matches = app.clone().get_matches();
@@ -401,6 +419,8 @@ fn main() {
         test_command(matches, &import_paths, cache, &registry, strict);
     } else if let Some(_) = app_matches.subcommand_matches("converters") {
         converters_command(&registry)
+    } else if let Some(_) = app_matches.subcommand_matches("env") {
+        env_help()
     } else {
         app.print_help().unwrap();
         println!("");