Resolve paths in config

[mrlubos]

This would take care of the issues reported with runtimeConfigPath for example, but we should apply this for all arguments accepting paths. Let's take an example

runtimeConfigPath: './src/hey-api.ts'

this should take the configFile path from c12

const { configFile } = await loadConfig();

and use it as the base, e.g. path.resolve(configFile, configOption). One thing to consider for the future is if the path comes from CLI, we want to resolve against cwd (?)