allow lua expressions in templates

[horriblename]

allows using lua expressions for string manipulation during replace. Example:

-- ORIGINAL TEXT:
local foo_bar = 3

-- SEARCH:
local $var = $val

-- REPLACE:
local $(snake_to_camel_case(var) .. val) = $val

-- RESULT:
local fooBar3 = 3

things to note:

• var and val are exposed as "readonly" global in the lua expression - but existing globals will take priority (lua limitation)
• snake_to_camel_case is user defined

this probably counts as a breaking change

I'm curious to see what others think - if you have suggestions let me know

[horriblename]

TODO:

• figure out error handling
• handle nil return
• how to handle indent?

nice to have:

• extend the standard string lib with common functions such as toCamelCase etc. would love to be able to write $(var:toCamelCase()) ootb
• allow swapping out for fennel. I don't use fennel myself but the pipe operator is so good here