Processing several input files by hurlfmt

[fabricereix]

Currently, hurlfmt takes only one input file.
It could be generalized to several input files, especially useful when reformatting Hurl file in-place.

with the default output (not in-place), it should simply concatenate all the output, like cat.

For example

file1.hurl
GET http://localhost:8000/hello

file2.hurl
GET http://localhost:8000/hi

hurlfmt file1.hurl file2.hurl
GET http://localhost:8000/hello
GET http://localhost:8000/hi

$ hurlfmt --out json file1.hurl file2.hurl
{"entries":[{"request":{"method":"GET","url":"http://localhost:8000/hello"}}]}
{"entries":[{"request":{"method":"GET","url":"http://localhost:8000/hi"}}]}