Colons in default values for environment variables

#63 argued about variable names that contained colons, and while that does seem like an unlikely use case, a variable's value that contains a colon is not that uncommon.

Example: `${localEnv:API_URL:https://api.modrinth.com/v2/}`

This will evaluate to `https`, whereas one would expect it'd evaluate to `https://api.modrinth.com/v2`, akin to `${API_URL:-https://api.modrinth.com/v2/}` in Bash.

Would be nice to have it in specification that any colon following the second colon will be preserved as is in the default value, rather than terminate the default value segment. That'd mean the above example would correctly evaluate to `https://api.modrinth.com/v2/`.