✨ Add long-form option parsing

So far we could only add single-letter (POSIX) options: `-e`, `-n` and
so forth. Since we will run out of alphabet at some point, this commit
introduces parsing of multi-letter long-form (gnu) options (plus any
modern application should really support it anyways):
`--echo`, `--noline`.

Additionally, we support supplying long-form options that supply a value
both in the spaced (`--hist-limit 0`) and the equals (`--hist-limit=0`)
forms. Short, long, spaced, equals can be mixed and matched between
freely.

Lastly, we retain the ability to concatenate short options as before
(`-ne` is valid, as is `-P0` for the respective options above).

This should cover all bases and does not complicate the code too much to
keep a coherent overview.

Changed several code samples in the documentation to make
use of short- or long-form options to point out
possibility.

test/bemoji_cmds.bats

@@ -85,3 +85,29 @@ typing result"
     BEMOJI_PICKER_CMD="echo heart" run --keep-empty-lines -- bemoji -ne
     assert_output --regexp '^heart$'
 }
+
+@test "Understands long-form options" {
+    run bemoji --help
+    assert_success
+    assert_output --partial "A simple emoji picker."
+}
+
+@test "Understands long-form equals values" {
+    BEMOJI_CLIP_CMD="echo heart" run bemoji --hist-limit=0
+    assert_success
+}
+
+@test "Understands long-form spaced values" {
+    BEMOJI_CLIP_CMD="echo heart" run bemoji --hist-limit 0
+    assert_success
+}
+
+@test "Understands short-form spaced values" {
+    BEMOJI_CLIP_CMD="echo heart" run bemoji -P 0
+    assert_success
+}
+
+@test "Can concatenate short-form options and values" {
+    BEMOJI_CLIP_CMD="echo heart" run bemoji -neP0
+    assert_success
+}