Syntax for formatting individual file is annoying

[araujoms]

If I just want to run Runic on the file test.jl, I need to do

using Runic
Runic.format_file("test.jl"; inplace = true)

For comparison, with JuliaFormatter I only need to do

using JuliaFormatter
format("test.jl")

Just export the function and let the default value for inplace be true.

[fredrikekre]

Since it is a (potentially) destructive operation I like the explicitness of requiring the inplace keyword argument. I also prefer not exporting names, but they can be marked with public in supported Julia versions (see #127).