Add --release flag

[smoelius]

I think five steps are necessary:

1. Modify the TestFuzzWithDeprecations struct to add the --release flag:

   test-fuzz/cargo-test-fuzz/src/bin/cargo_test_fuzz/transition.rs

   Line 26 in 42bacfd

   struct TestFuzzWithDeprecations {

2. Add a release field to this TestFuzz struct:

   test-fuzz/cargo-test-fuzz/src/lib.rs

   Line 74 in 42bacfd

   pub struct TestFuzz {

3. Modify this From implementation to set the release field in the TestFuzz struct when that field is set in the TestFuzzWithDeprecations struct:

   test-fuzz/cargo-test-fuzz/src/bin/cargo_test_fuzz/transition.rs

   Line 133 in 42bacfd

   impl From<TestFuzzWithDeprecations> for super::TestFuzz {

4. Modify the build function so that it checks whether the release field is set, and adds --release to the flags passed to cargo when it is:

   test-fuzz/cargo-test-fuzz/src/lib.rs

   Line 231 in 42bacfd

   fn build(opts: &TestFuzz, quiet: bool) -> Result<Vec<Executable>> {

5. Update the "Usage" section in the README: https://github.com/trailofbits/test-fuzz/blob/master/README.md#usage