Allow for a simpler way to set system instructions

[peterfriese]

This change allows developers to write

model = GenerativeModel(name: "gemini-1.0-pro", apiKey: APIKey.default, systemInstruction: "Talk like a pirate")

instead of

model = GenerativeModel(name: "gemini-1.0-pro", apiKey: APIKey.default, systemInstruction: ModelContent(role: "system", parts: "Talk like a pirate"))

[andrewheard]

Thanks, Peter!

[andrewheard]

Since this isn't a breaking change, I think it would be reasonable to do a quick 0.5.1 patch to make sure our new docs will match the latest release. So I wouldn't forget, I updated the version in #150.