Update documentation for reports, new CLI

This is basically a redraft of the documentation to be more complete and
better cover some of the features added since it was written.

docs/src/config-file.md

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+# Configuration
+
+The `task` command will work out-of-the-box with no configuration file, using default values.
+
+Configuration is read from `taskchampion.yaml` in your config directory.
+On Linux systems, that directory is `~/.config`.
+On OS X, it's `~/Library/Preferences`.
+On Windows, it's `AppData/Roaming` in your home directory.
+The path can be overridden by setting `$TASKCHAMPION_CONFIG`.
+
+Individual configuration parameters can be overridden by environment variables, converted to upper-case and prefixed with `TASKCHAMPION_`, e.g., `TASKCHAMPION_DATA_DIR`.
+Nested configuration parameters such as `reports` cannot be overridden by environment variables.
+
+## Directories
+
+* `data_dir` - path to a directory containing the replica's task data (which will be created if necessary).
+  Default: `taskchampion` in the local data directory.
+
+## Sync Server
+
+If using a local server:
+
+* `server_dir` - path to a directory containing the local server's data.
+  This is only used if `server_origin` or `server_client_key` are not set.
+  Default: `taskchampion-sync-server` in the local data directory.
+
+If using a remote server:
+
+* `server_origin` - Origin of the TaskChampion sync server, e.g., `https://taskchampion.example.com`.
+  If not set, then sync is done to a local server.
+* `encryption_secret` - Secret value used to encrypt all data stored on the server.
+  This should be a long random string.
+  If you have `openssl` installed, a command like `openssl rand -hex 35` will generate a suitable value.
+  This value is only used when synchronizing with a remote server -- local servers are unencrypted.
+  Treat this value as a password.
+* `server_client_key` -  Client key to identify this replica to the sync server (a UUID)
+  If not set, then sync is done to a local server.
+
+# Reports
+
+* `reports` - a mapping of each report's name to its definition.
+  See [Reports](./reports.md) for details.