Refactor integration tests (#71)

- Create subcommands to be able to print modules independently
	- `starship prompt` will print the full prompt
	- `starship module <MODULE_NAME>` will print a specific module
		e.g. `starship module python`
	- Added `--path` flag to print the prompt or modules without being in a specific directory
	- Added `--status` flag to provide the status of the last command, instead of requiring it as an argument
- Refactored integration tests to be end-to-end tests, since there was no way in integration tests to set the environment variables for a specific command, which was required for the `username` module
- Moved e2e tests to `tests/testsuite` to allow for a single binary to be built
	- Tests will build/run faster
	- No more false positives for unused functions
- Added tests for `username`
- Removed codecov + tarpaulin 😢

src/context.rs

@@ -15,11 +15,15 @@ pub struct Context<'a> {
 
 impl<'a> Context<'a> {
     pub fn new(arguments: ArgMatches) -> Context {
-        let current_dir = env::current_dir().expect("Unable to identify current directory.");
-        Context::new_with_dir(arguments, current_dir)
+        // Retreive the "path" flag. If unavailable, use the current directory instead.
+        let path = arguments
+            .value_of("path")
+            .map(From::from)
+            .unwrap_or_else(|| env::current_dir().expect("Unable to identify current directory."));
+
+        Context::new_with_dir(arguments, path)
     }
 
-    #[allow(dead_code)]
     pub fn new_with_dir<T>(arguments: ArgMatches, dir: T) -> Context
     where
         T: Into<PathBuf>,