Add a mechanism for building a cache of "system parameters"

- Add a system_parameters module to eryntools that implements some basic types and can write a cache to a file.
- Add a system-parameters Python script that builds the cache.
- Add init_system_parameters to zsh that exports an environment variable that points to the parameters file
- Add an eryn.system-parameters.plist LaunchAgent file for running the script at regular intervals

bin/system-parameters

@@ -0,0 +1,41 @@
+#!/usr/bin/env python3
+
+import argparse
+from eryntools.dotfiles.system_parameters import CommandParameter, SystemParameters
+
+
+class HomebrewPrefix(CommandParameter):
+    def __init__(self):
+        super().__init__(['brew', '--prefix'])
+
+
+class XcodeSelectPrintPath(CommandParameter):
+    def __init__(self):
+        super().__init__(['xcode-select', '-p'])
+
+
+def main(argv):
+    parser = argparse.ArgumentParser(prog=argv[0])
+    parser.add_argument('-f', '--file', type=argparse.FileType('r'))
+    parser.add_argument('-o', '--output', type=argparse.FileType('w'), default=sys.stdout)
+    arguments = parser.parse_args(argv[1:])
+
+    output_parameters = SystemParameters()
+
+    if arguments.file:
+        output_parameters.load_from_file(arguments.file)
+
+    parameters = {
+        'homebrew_prefix': HomebrewPrefix(),
+        'xcode_path': XcodeSelectPrintPath(),
+    }
+
+    for name, parameter in parameters.items():
+        setattr(output_parameters, name, parameter)
+
+    output_parameters.write_to_file(arguments.output)
+
+
+if __name__ == "__main__":
+    import sys
+    sys.exit(main(sys.argv) or 0)