Proper loading of 3rd party rulesets and reporters.

[Tapchicoma]

Unblocks using them on on java 9+.

Base on ideas discussed in #495 and #573 PRs.

To test - execute following steps:

$ ./gradlew :ktlint-ruleset-template:jar
...
$ ./gradlew sJE
...
$ docker run --rm -ti -w "/command" -v `pwd`:/command openjdk:11-jdk-slim ./ktlint/build/run/ktlint --debug --experimental --ruleset=ktlint-ruleset-template/build/libs/ktlint-ruleset-template.jar
[DEBUG] Discovered ruleset with "standard" id.
[DEBUG] Discovered ruleset with "custom" id.
[DEBUG] Discovered ruleset with "experimental" id.
[DEBUG] Discovered reporter with "checkstyle" id.
[DEBUG] Discovered reporter with "json" id.
[DEBUG] Discovered reporter with "plain" id.
[DEBUG] Initializing "plain" reporter with {verbose=false, color=false}
[DEBUG] Checking /command/ktlint/src/main/kotlin/com/pinterest/ktlint/Main.kt
...

You should see that ruleset with "custom" id is discovered and lint failed with a lot of errors "Unexpected var, use val instead (cannot be auto-corrected)".

Closes #259.

[shashachu]

thank you!