init: allow whitespaces in package extras (#2985)

This fix allows having whitespaces in the list of extras during poetry 
add and init, e.g. `databases[postgresql, sqlite]`.

Resolves: #2980

poetry/console/commands/init.py

@@ -373,7 +373,7 @@ The <c1>init</c1> command creates a basic <comment>pyproject.toml</> file in the
         for requirement in requirements:
             requirement = requirement.strip()
             extras = []
-            extras_m = re.search(r"\[([\w\d,-_]+)\]$", requirement)
+            extras_m = re.search(r"\[([\w\d,-_ ]+)\]$", requirement)
             if extras_m:
                 extras = [e.strip() for e in extras_m.group(1).split(",")]
                 requirement, _ = requirement.split("[")

tests/console/commands/test_init.py

@@ -556,3 +556,12 @@ python = "~2.7 || ^3.6"
     assert expected in output
     assert 'pytest-requests = "^0.2.0"' in output
     assert 'pytest = "^3.6.0"' in output
+
+
+def test_add_package_with_extras_and_whitespace(tester):
+    result = tester._command._parse_requirements(["databases[postgresql, sqlite]"])
+
+    assert result[0]["name"] == "databases"
+    assert len(result[0]["extras"]) == 2
+    assert "postgresql" in result[0]["extras"]
+    assert "sqlite" in result[0]["extras"]