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 😢

integration_test

@@ -6,9 +6,6 @@ if ! (docker --version); then
   exit 1
 fi
 
-printf 'Pulling latest docker image'
-docker pull starshipcommand/starship-test
-
 printf 'Building test docker image:\n'
 docker build -f tests/Dockerfile \
   --tag starshipcommand/starship-test \
@@ -16,6 +13,4 @@ docker build -f tests/Dockerfile \
   .
 
 printf 'Running test suite:\n'
-# `seccomp=unconfined` is needed to allow tarpaulin to disable ASLR
-# Details found here: https://github.com/xd009642/tarpaulin/issues/146
-docker run --rm --security-opt seccomp=unconfined -v $(pwd):/starship starshipcommand/starship-test
+docker run --rm -v $(pwd):/starship starshipcommand/starship-test