Add tracks flag to subjects command

[doneill]

• Add subject tracks flag
  • Print to screen or export to file
• Add tracks flag test to subjects

er subjects --help
Return subject data and tracks updated within specified number of days ago

Usage:
  er subjects [flags]

Flags:
  -e, --export              Export tracks to GeoJSON file
  -h, --help                help for subjects
  -s, --subject-id string   Subject ID for tracks query
  -t, --tracks              Get tracks for a subject
  -u, --updated-since int   Number of days ago to query updates from (default 3)

> er subjects -s [SUBJECT_ID] -t -u 3 -e
Successfully exported tracks to [SUBJECT_ID].geojson