Check variable names for RF's recommendation

[MoreFamed]

RF's documentation reads: Variable names can basically have any characters between the curly braces. However, using only alphabetic characters from a to z, numbers, underscore and space is recommended, and it is even a requirement for using the extended variable syntax. This is why I suggest to create a rule for checking it: variable name should match [a-zA-Z0-9_ ]+

Note: rule hyphen-in-variable-name could be deleted, rules checking upper/lowercase in variable names could be left untouched.