Support (part of) the XDG Base Directory Specification (#920)

On Unix-like systems, the configuration file now lives in
$XDG_CONFIG_HOME/httpie/ by default, not ~/.httpie/ (the behaviour on
Windows is unchanged). The previous location is still checked, in order
to support existing installations.

Searching $XDG_CONFIG_DIRS is still not supported.

Fixes #145; supersedes #436.

tests/test_config.py

@@ -1,7 +1,10 @@
+import os.path
+from pathlib import Path
+
 import pytest
 
 from httpie.compat import is_windows
-from httpie.config import Config
+from httpie.config import Config, get_default_config_dir
 from utils import HTTP_OK, MockEnvironment, http
 
 
@@ -48,3 +51,40 @@ def test_default_options_overwrite(httpbin):
     assert r.json['json'] == {
         "foo": "bar"
     }
+
+
+@pytest.mark.skipif(is_windows, reason='XDG_CONFIG_HOME is only supported on *nix')
+def test_config_file_location_xdg(monkeypatch, tmp_path):
+    monkeypatch.delenv('HTTPIE_CONFIG_DIR', raising=False)
+    home = tmp_path.absolute().as_posix()
+    monkeypatch.setenv('HOME', home)
+    xdg_config_home = tmp_path.joinpath("different_config")
+    os.mkdir(xdg_config_home)
+    monkeypatch.setenv('XDG_CONFIG_HOME', xdg_config_home.absolute().as_posix())
+    assert get_default_config_dir() == xdg_config_home.joinpath('httpie')
+    monkeypatch.delenv('XDG_CONFIG_HOME')
+    # NB: this should be true even though .config doesn't exist.
+    assert get_default_config_dir() == tmp_path.joinpath('.config', 'httpie')
+    monkeypatch.setenv('XDG_CONFIG_HOME', 'some/nonabsolute/path')
+    assert get_default_config_dir() == tmp_path.joinpath('.config', 'httpie')
+
+
+@pytest.mark.skipif(is_windows, reason='legacy config file location is only checked on *nix')
+def test_config_file_location_legacy(monkeypatch, tmp_path):
+    monkeypatch.delenv('HTTPIE_CONFIG_DIR', raising=False)
+    home = tmp_path.absolute().as_posix()
+    monkeypatch.setenv('HOME', home)
+    os.mkdir(tmp_path.joinpath('.httpie'))
+    assert get_default_config_dir() == tmp_path.joinpath('.httpie')
+
+
+@pytest.mark.skipif(not is_windows, reason='windows-only')
+def test_config_file_location_windows(monkeypatch):
+    monkeypatch.delenv('HTTPIE_CONFIG_DIR', raising=False)
+    assert get_default_config_dir() == Path(os.path.expandvars(r'%APPDATA%\httpie'))
+
+
+def test_config_file_location_custom(monkeypatch, tmp_path):
+    httpie_config_dir = tmp_path.joinpath('custom', 'directory')
+    monkeypatch.setenv('HTTPIE_CONFIG_DIR', str(httpie_config_dir.absolute()))
+    assert get_default_config_dir() == httpie_config_dir