Add better support for other static site generators

Currently, Staticman will look for a _config.yml configuration file on the root of the repository, which is specific to Jekyll.

I’d like us to support other engines, so I see two options:

1. Allow requests to specify the path to the configuration file within the options object (ugly, potential security issues?)
2. Move the staticman config block to its own file

Option 2 seems more appropriate, but what should the name for the new config file be? Jekyll doesn’t include files prefixed with an underscore in the generated site — is this a convention used by other engines as well? If not, is there a filename we could use that would cause this effect in all (or, at least, in the majority) of engines?

@biilmann would you be able to advise on this?

(As a side note, this will be a breaking change, so I’ll have to release a v2endpoint for entries)

[eduardoboucas]

Oh, wow. That’s ugly indeed.

I’m thinking the new config file could simply be staticman.yml in the root of the repository. For simplicity reasons, we need to support a single format for the config file, and YAML seems to be the most human-friendly choice.

[eduardoboucas]

Closing in favour of #32. Will release with v2 of the API shortly.