support for single line switches in ini files

[GeoffWilliams]

This PR allows the library to support single line switches in INI files (Addresses #41)

PR Highlights:

• single words made up of valid word characters are parsed as a switch
• switches parse as an empty hash {} and this can be used when saving the file back
• the empty hash was used instead of a simple Boolean value to avoid clashing with real values
• likewise nil clashes with the empty string ''
• tests added
• documentation updated