Only show violated rules when target version is given

When the --target argument is given, we expect vermin to work more like a linter: it has a predefined goal, matches some rules and report violations (and set exit code to 0 or 1 accordingly). Typically linters emit as few output lines as possible when there is no violation found, and only display violated rules when found. This helps users spot rule violations more easily, compare this:

$ python3 vermin.py -t=2.7 -t=3.0 --format parsable .
/path/to/vermin/misc/travis/after_success.sh:4:7: error: invalid syntax: if [[ $TRAVIS_PYTHON_VERSION != 2.7 && $TRAVIS_PYTHON_VERSION != 3.4 ]]; then
/path/to/vermin/misc/travis/script.sh:4:7: error: invalid syntax: if [[ $TRAVIS_PYTHON_VERSION = 2.7 || $TRAVIS_PYTHON_VERSION = 3.4 ]]; then
(many lines omitted ...)
/path/to/vermin/runtests.py:8::2.7:3.1:'unittest.main(exit)'
/path/to/vermin/runtests.py:::2.7:3.1:
(many lines omitted ...)
:::2.7:3.1:

with this:

$ python3 vermin.py -t=2.7 -t=3.0 --format parsable .
/path/to/vermin/runtests.py:8::2.7:3.1:'unittest.main(exit)'
/path/to/vermin/runtests.py:::2.7:3.1:
:::2.7:3.1:

We can either:

• Add a flag like --only-show-violated (maybe a shorter name) to do this, this flag only makes sense when --target is given
• Make this behavior to be the default when --target is given (this is backwards incompatible behavior change)

[gousaiyang]

I’ve tested this against some of my projects and I think the current implementation is pretty good.

[gousaiyang]

-X is probably a bit weird since it usually means “eXtension” or “eXtra” or “eXecute”.