Custom keys for custom provisioning in vvv-init.sh

[tomjn]

The new vvv-config.yml/vvv-custom.yml allows you to specify a site or sites, with various keys. However, a vvv-init.sh may want to specify custom keys that are too specific to apply to all VVV sites.

This PR installs python-pip and the shyaml dependency, and adds a bash function named customvalue for retrieving those keys. When the custom section or the requested key are not defined or found, the function will return nothing

E.g. in vvv-init.sh

colour=`customvalue 'colour' 'pink'`
echo "Hello there :) This site is a ${colour} site"

Which when ran will print Hello there :) This site is a pink site, and can be changed as follows in vvv-custom.yml:

  redsite:
    custom:
      colour: red

Resulting in Hello there :) This site is a red site

Nested keys can be accessed using dot syntax, e.g.:

  redsite:
    custom:
      colour:
        primary: red
        secondary: blue

primarycolour=`customvalue 'colour.primary' 'pink'`
echo "Hello there :) This site is a ${primarycolour} site"

[jeremyfelt]

This is really slick, thanks @tomjn!

I'd like to revisit the name customvalue as I think it can be more descriptive. How about something like get_config_key() or get_yaml_option? Happy to hear any other ideas as well.

[tomjn]

I like get_config_key, an obvious next step in functionality here would be to return values rather than a singular value, so get_config_value makes more sense, as it leads to the get_config_values function

[tomjn]

I've renamed the function to get_config_value

[jeremyfelt]

Great stuff, thanks @tomjn!