Add LICENSE. Add take_any_args.

LICENSE

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+The MIT License
+
+Copyright (c) 2019 Arthur Sonzogni.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+

ftxui/include/ftxui/dom/take_any_args.hpp

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+#include <type_traits>
+
+// Turn a set of arguments into a vector.
+template <class... Args>
+Children unpack(Args... args) {
+  using T = std::common_type_t<Args...>;
+  std::vector<T> vec;
+  (vec.push_back(std::forward<Args>(args)), ...);
+  return vec;
+}
+
+// Make |container| able to take any number of argments.
+#define TAKE_ANY_ARGS(container) \
+  template <class... Args> \
+  Element container(Args... children) { \
+    return container(unpack(std::forward<Args>(children)...)); \
+  } \
+