Add Conan support

[janekb04]

Regarding glfwpp-conan by @Ahajha.

[Ahajha]

Hey janekb04, I'm glad you're interested in Conan support. I have a few questions/requests/comments regarding how we could move forward with the recipe.

1. Conan is generally intended to be used with tagged releases. Right now I just have the recipe pointing to latest. Since this library seems to have hit a point of stability, I would perhaps recommend tagging the current state as version 1.0.0 (or 0.1.0).
2. We could either merge the Conan recipe into this repo, or I could open an MR in conan-center-index directly. In the latter case, users wouldn't need to manually install the recipe before using it in a project. It would take a bit longer before it's ready, maybe a week or two.
3. If you want to just include the recipe in this repo, I could restructure it differently so that it just includes the files as part of the recipe instead of cloning it.
4. As far as dependencies go, this (obviously) needs glfw, which I'm using 3.3.6 (since that's the latest version you specifically state in the README). I'm not certain how the others work, they all seem optional. As far as I can tell you have imgui and glew, not sure what the versions are.

Let me know your thoughts!