feat: jless default opts env

[tronikelis]

Problem

There is no way to set preferred default config flags.

Solution

I propose to add a new env variable JLESS_DEFAULT_OPTS inspired by FZF_DEFAULT_OPTS which doesn't require to choose a new config file format (json/yaml/toml) and allows configuration just as you would type the config in your shell.

For example:

export JLESS_DEFAULT_OPTS="-r"
# now jless will launch with relative numbers by default
jless

Notes

Current implementation does not handle config properties with spaces in them
JLESS_DEFAULT_OPTS="--file \"foo bar\"" wouldn't work

closes #174