hl7: name the unexpected segment error, allow segment to be extracted

hl7: add option to ignore check for separator field runes in text fields

hl7: allow round trip ACK triggers

decode: support decoding when field level errors are present

hl7: prevent panic in parse date time, be more robust

Closes #4

support true VARIES segment

decode only look at first sub-field when parsing date time

hl7: prevent panic on malformed hl7 message, adjust error message

hl7: add remaining versions

add h230, h270, h271