build: reorganize requirements, mark hidapi as optional

This drops the command line options. `libusb` is now required
unconditionally (it's cffi so no harm there). hidapi is an extra.
You either need to install it manually, or specify it in your
requirements.txt like so:

trezor[hidapi] >= 0.9.2

requirements-dev.txt

@@ -0,0 +1,4 @@
+-r requirements.txt
+-r requirements-optional.txt
+pytest>=3.5.0
+mock>=2.0.0

requirements-optional.txt

@@ -0,0 +1 @@
+hidapi >= 0.7.99.post20

requirements.txt

@@ -3,6 +3,5 @@ mnemonic>=0.17
 requests>=2.4.0
 click>=6.2
 pyblake2>=0.9.3
-hidapi>=0.7.99.post20
 libusb1>=1.6.4
 rlp>=0.6.0

setup.py

@@ -15,19 +15,9 @@ install_requires = [
     'click>=6.2',
     'pyblake2>=0.9.3',
     'rlp>=0.6.0',
+    'libusb1>=1.6.4',
 ]
 
-import sys
-if '--disable-hidapi' in sys.argv:
-    sys.argv.remove('--disable-hidapi')
-else:
-    install_requires.append('hidapi>=0.7.99.post20')
-
-if '--disable-libusb' in sys.argv:
-    sys.argv.remove('--disable-libusb')
-else:
-    install_requires.append('libusb1>=1.6.4')
-
 from trezorlib import __version__ as VERSION
 
 
@@ -97,6 +87,9 @@ setup(
     },
     scripts=['trezorctl'],
     install_requires=install_requires,
+    extras_require={
+        'hidapi': ['hidapi>=0.7.99.post20'],
+    },
     python_requires='>=3.3',
     include_package_data=True,
     zip_safe=False,