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 😢

tests/testsuite/common.rs

@@ -0,0 +1,22 @@
+use std::{io, process};
+
+pub fn render_prompt() -> process::Command {
+    let mut command = process::Command::new("./target/debug/starship");
+    command.arg("prompt");
+
+    command
+}
+
+pub fn render_module(module_name: &str) -> process::Command {
+    let mut command = process::Command::new("./target/debug/starship");
+    command.arg("module").arg(module_name);
+
+    command
+}
+
+/// Create a temporary directory with full access permissions (rwxrwxrwt).
+pub fn new_tempdir() -> io::Result<tempfile::TempDir> {
+    //  Using `tempfile::TempDir` directly creates files on macOS within
+    // "/var/folders", which provides us with restricted permissions (rwxr-xr-x)
+    tempfile::tempdir_in("/tmp")
+}