Add setup option to configure custom file assets

Resolve #11

lua/presence/file_assets.lua

@@ -1,4 +1,15 @@
--- Discord application asset file names (name, key[, description]) by file extension
+-- File asset definitions (name, source) keyed by file name or extension
+-- * name - the name of the asset shown as the title of the file in Discord
+-- * source - the source of the asset, either an art asset key or the URL of an image asset
+--
+-- Example: {
+--     -- Use art assets uploaded in Discord application for the configured client id
+--     js = { "JavaScript", "javascript" },
+--     ts = { "TypeScript", "typescript" },
+--     -- Use image URLs
+--     rs = { "Rust", "https://www.rust-lang.org/logos/rust-logo-512x512.png" },
+--     go = { "Go", "https://go.dev/blog/go-brand/Go-Logo/PNG/Go-Logo_Aqua.png" },
+-- }
 return {
     [".aliases"] = { ".aliases", "shell" },
     [".appveyor.yml"] = { "AppVeyor config", "appveyor" },