Allow partial rustfmt.toml
Allow partial rustfmt.toml

With this change one can use a config file
that only specifies a subset of config keys
to overwrite. E.g. a config file that looks like this

struct_trailing_comma = "Never"
struct_lit_trailing_comma = "Never"

Fixes #255

aidancully/rust,SingingTree/rustfmt,graydon/rust,SingingTree/rustfmt,rust-lang/rustfmt,graydon/rust,aidancully/rust,studoot/rustfmt,rust-lang/rustfmt,flutterhash/rustfmt,aidancully/rust,graydon/rust,graydon/rust,Emilgardis/rustfmt,Emilgardis/rustfmt,studoot/rustfmt,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust