add mechanism to pass jinja_filters

[dbarrosop]

For instance:

# jinja_filters/__init__.py

def get_filters():
    return {"test": test}


def test(data):
    return "yes!!!"

and then:

    brg = InitBrigade(
        dry_run=dry_run,
        SimpleInventory={"host_file": HOSTS_FILE, "group_file": GROUPS_FILE},
        jinja_filters="jinja_filters.get_filters",
    )

[dbarrosop]

Solves #94

[coveralls]

Pull Request Test Coverage Report for Build 536

• 19 of 20 (95.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-4.2%) to 87.363%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
brigade/core/configuration.py	8	9	88.89%

Totals
Change from base Build 534:	-4.2%
Covered Lines:	878
Relevant Lines:	1005

---

💛 - Coveralls

[ogenstad]

Looks good.