Don't depend on pwd

[wellle]

Currently elastex only works in directories that contain a config.yml. To make it work everywhere there are a couple of possible solutions:

• try in this order: ./.elastexrc, ~/.elastexrc, /etc/elastexrc
• add flag --config myconfigfile
• use a environment variable like $ELASTEXRC

[ferki]

The configuration behaviour is now documented on the (Configuring elastex)[https://github.com/adjust/elastex/wiki/Configuring-elastex] wiki page.

Note: for now it still tries to look for ./config.yml first for backwards compatibility, but we can deprecate that in favor of ./.elastexrc.