Add journal features

[hauleth]

• [x] Formatter for journal
• [x] Journal datagram handler
• [x] Journal stream handler
• [ ] Overload handler for handler

Close #5

[codecov[bot]]

Codecov Report

Merging #7 into master will increase coverage by 2.14%. The diff coverage is 97.31%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage   93.23%   95.37%   +2.14%     
==========================================
  Files           6        8       +2     
  Lines         133      281     +148     
==========================================
+ Hits          124      268     +144     
- Misses          9       13       +4

Impacted Files	Coverage Δ
src/systemd_stderr_formatter.erl	100% <ø> (ø)
src/systemd_journal_formatter.erl	100% <100%> (ø)
src/systemd_socket.erl	80% <100%> (ø)	:arrow_up:
src/systemd_journal_h.erl	82.6% <82.6%> (ø)

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