add support for inputting a text file

[mohsenasm]

Hi,
There are three small changes:

1. I added support for inputting a text file with a -sf /path/to/file flag. A use case for this flag is loading certificate files, which have new lines in their texts. So it may be inconvenient to load them with the -s flag. I have tested the new flag and it's working.
2. I update the readme to introduce this flag.
3. The install command in the readme needed to be changed (from go get to go install) as since Go 1.18 installing packages with go get is no longer supported.

[davidnewhall]

Changing an author's license file seems like a good way to get your contribution ignored.
This worked for me to obfuscate an SSL private key file; using bash where secret.key is the file name.

mumbojumbo -s "$(<secret.key)" -p sslkeys