Add support for more Unicode characters

When attempting to parse an iCalendar ICS export from Google, I discovered a few Unicode
code points in event `DESCRIPTION`s that were breaking the parser.

The three that I directly observed that broke the parser:
- U+0009 Horizontal Tab - https://unicode.org/cldr/utility/character.jsp?a=0009
- U+200B Zero Width Space - https://unicode.org/cldr/utility/character.jsp?a=200B
- U+00AD Soft Hyphen - https://unicode.org/cldr/utility/character.jsp?a=00ad

Based on my understanding, according to RFC5545[0], these characters should be supported.
However, `unicode.IsGraphic` does not return true for them.

This may be a hacky way to support it, but I think supporting all non-control unicode Characters is
closer to the spec without being too verbose/complex.

---

Rationale for support:

contentline   = name *(";" param ) ":" value CRLF
value         = *VALUE-CHAR
VALUE-CHAR    = WSP / %x21-7E / NON-US-ASCII
NON-US-ASCII  = UTF8-2 / UTF8-3 / UTF8-4

- U+009 is defined in WSP/HTAB in RFC5234 https://tools.ietf.org/html/rfc5234
- U+200B/U+00AD is defined by UTF8-2/UTF8-3/UTF8-4 in RFC3629 https://tools.ietf.org/html/rfc3629

fixtures/example.ics

@@ -12,4 +12,37 @@ CATEGORIES:CONFERENCE
 SUMMARY:Networld+Interop Conference
 DESCRIPTION:Networld+Interop Conference and Exhibit\nAtlanta World Congress Center\n Atlanta\, Georgia
 END:VEVENT
+BEGIN:VEVENT
+DTSTAMP:19960704T120000Z
+UID:uid1@example.com
+ORGANIZER:mailto:jsmith@example.com
+DTSTART:19960918T143000Z
+DTEND:19960920T220000Z
+STATUS:CONFIRMED
+CATEGORIES:CONFERENCE
+SUMMARY:Contains a tab
+DESCRIPTION:between the carets >	< is a horizontal tab
+END:VEVENT
+BEGIN:VEVENT
+DTSTAMP:19960704T120000Z
+UID:uid1@example.com
+ORGANIZER:mailto:jsmith@example.com
+DTSTART:19960918T143000Z
+DTEND:19960920T220000Z
+STATUS:CONFIRMED
+CATEGORIES:CONFERENCE
+SUMMARY:Contains a zero width space
+DESCRIPTION:between the carets >​< is a zero with space
+END:VEVENT
+BEGIN:VEVENT
+DTSTAMP:19960704T120000Z
+UID:uid1@example.com
+ORGANIZER:mailto:jsmith@example.com
+DTSTART:19960918T143000Z
+DTEND:19960920T220000Z
+STATUS:CONFIRMED
+CATEGORIES:CONFERENCE
+SUMMARY:Contains a soft hyphen
+DESCRIPTION:between the carets >­< is a soft hyphen
+END:VEVENT
 END:VCALENDAR

lex.go

@@ -346,7 +346,7 @@ func lexValue(l *lexer) stateFn {
 Loop:
 	for {
 		switch r := l.next(); {
-		case unicode.IsGraphic(r):
+		case isValueChar(r):
 			// absorb
 		default:
 			l.backup()
@@ -372,6 +372,10 @@ func isSafeChar(r rune) bool {
 	return !unicode.IsControl(r) && r != '"' && r != ';' && r != ':' && r != ','
 }
 
+func isValueChar(r rune) bool {
+	return r == '\t' || (!unicode.IsControl(r) && utf8.ValidRune(r))
+}
+
 // item helpers
 
 // isItemName checks if the item is an ical name