Fetch: improve jql integration

[mhuang74]

Improve JQL integration for Fetch (#77)

• added better error messaging for jql errors
• added support for multi-value selectors by concatenating them into comma separated string (e.g. "Anaheim, CA")
• added support for JSON Number types properly
• added usage examples in Fetch.md
• refactored and added unit tests for jql integration
• added integration test for multi-value jql selector
• added errors for unsupported CLI params

mhuang@twisted-linen:~/Projects/rust/qsv (fetch-fix-jql)$ cargo test fetch
    Finished test [optimized + debuginfo] target(s) in 0.29s
     Running unittests (target/debug/deps/qsv-018ebfd29fc3825d)

running 5 tests
test cmd::fetch::test_apply_jql_null ... ok
test cmd::fetch::test_apply_jql_bool ... ok
test cmd::fetch::test_apply_jql_array ... ok
test cmd::fetch::test_apply_jql_number ... ok
test cmd::fetch::test_apply_jql_string ... ok

test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running unittests (target/debug/deps/qsvlite-51c40fd03267f4fc)

running 5 tests
test cmd::fetch::test_apply_jql_null ... ok
test cmd::fetch::test_apply_jql_bool ... ok
test cmd::fetch::test_apply_jql_number ... ok
test cmd::fetch::test_apply_jql_array ... ok
test cmd::fetch::test_apply_jql_string ... ok

test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/tests.rs (target/debug/deps/tests-df10d0df673cea23)

running 4 tests
test test_fetch::fetch_jql_single ... ok
test test_fetch::fetch_jql_multiple ... ok
test test_fetch::fetch_simple ... ok
test test_fetch::fetch_ratelimit ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 616 filtered out; finished in 9.03s