Add [fetch-content].subdir

Closes #98

docs/cmake-toml.md

@@ -180,6 +180,7 @@ git = "https://github.com/myuser/gitcontent"
 tag = "v0.1"
 shallow = false
 system = false
+subdir = ""
 
 [fetch-content.svncontent]
 condition = "mycondition"
@@ -195,6 +196,8 @@ hash = "SHA1 502a4e25b8b209889c99c7fa0732102682c2e4ff"
 sha1 = "502a4e25b8b209889c99c7fa0732102682c2e4ff"
 ```
 
+Table keys that match CMake variable names (`[A-Z_]+`) will be passed to the [`FetchContent_Declare`](https://cmake.org/cmake/help/latest/module/FetchContent.html#command:fetchcontent_declare) command.
+
 ## Targets
 
 ```toml

include/project_parser.hpp

@@ -156,6 +156,7 @@ struct Content {
     ConditionVector include_before;
     ConditionVector include_after;
     bool system;
+    std::string subdir;
 };
 
 enum MsvcRuntimeType {

src/project_parser.cpp

@@ -507,6 +507,8 @@ Project::Project(const Project *parent, const std::string &path, bool build) : p
                     value = algo + "=" + value;
                 } else if (key == "hash") {
                     key = "URL_HASH";
+                } else if (key == "subdir") {
+                    key = "SOURCE_SUBDIR";
                 } else if (is_cmake_arg(key)) {
                     // allow passthrough of ExternalProject options
                 } else if (!c.visisted(key)) {