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/Dockerfile

@@ -1,9 +1,5 @@
 FROM rust
 
-# Install Tarpaulin (code coverage tool)
-# https://github.com/xd009642/tarpaulin
-COPY --from=xd009642/tarpaulin /usr/local/cargo/bin/cargo-tarpaulin /usr/local/cargo/bin/cargo-tarpaulin
-
 # Install Node.js
 ENV NODE_VERSION 12.0.0
 ENV PATH /root/.nvm/versions/node/v$NODE_VERSION/bin:$PATH
@@ -54,5 +50,4 @@ RUN rm -rf /starship
 RUN mkdir starship
 WORKDIR /starship
 
-# Instrument coverage tooling and run the full test suite
-CMD ["cargo", "tarpaulin", "--ignored", "--out", "Xml"]
+CMD ["cargo", "test", "--", "--ignored"]