plain C implementation of jmsh/bmsh output, -d 0/1 for output precision, accept .jnii input

[fangq]

hi @neurolabusc, this PR includes the following additions:

1. re-implemented save_jmsh with plain C instead of cJSON, cutting down the binary size
2. added binary JMesh (.bmsh) output
3. added -d 0/1 to control output precision, allow double-precision output for jmsh/bmsh/json output formats
4. using make JSON=1, it can mesh JNIfTI (.jnii) based volumes, including those output by dcm2niix -e j enabled with this patch. a compact example .jnii file is added.

the nii2mesh binary generated by make is 152.7kB, and that by make JSON=1 is 180.8kB.