Introduce integrated package manager

This commit introduces an integrated package manager into TShock
for the purpose of fetching and installing plugins and their dependencies
from NuGet repositories.

This makes getting new plugins easier for users, as well as simplifiying
more advanced deployment scenarios.

docs/packages-help.txt

@@ -0,0 +1,13 @@
+Description:
+  Manage plugins and their requirements
+
+Usage:
+  TShock.Server [command] [options]
+
+Options:
+  --version       Show version information
+  -?, -h, --help  Show help and usage information
+
+Commands:
+  sync  Install the plugins as specified in the plugins.json
+