As a carryover from the days when we only allowed one source, the source
field was still labeled 'source' instead of 'sources'. Fixed and updated
all of the tests.
This adds code to parse TOML files into Values maps. These can then
easily be passed into the template engine.
Included in this is support for TOML "tables", subsections of TOML
files. We will be using those to pass config data to dependent charts.
