Allow configuration of shell used for shell out

[denisidoro]

Benchmark #1: echo -e 'foo bar\nlorem ipsum' | NAVI_SHELL=dash navi fn map::expand
  Time (mean ± σ):      17.7 ms ±   4.8 ms    [User: 3.6 ms, System: 10.2 ms]
  Range (min … max):    10.9 ms …  38.1 ms    91 runs
 
Benchmark #1: echo -e 'foo bar\nlorem ipsum' | NAVI_SHELL=bash navi fn map::expand
  Time (mean ± σ):      22.0 ms ±   4.6 ms    [User: 5.2 ms, System: 7.9 ms]
  Range (min … max):    14.7 ms …  37.6 ms    88 runs
 
Benchmark #1: echo -e 'foo bar\nlorem ipsum' | NAVI_SHELL=sh navi fn map::expand
  Time (mean ± σ):      25.6 ms ±   5.5 ms    [User: 4.6 ms, System: 13.1 ms]
  Range (min … max):    17.6 ms …  40.3 ms    77 runs
 
Benchmark #1: echo -e 'foo bar\nlorem ipsum' | NAVI_SHELL=fish navi fn map::expand
  Time (mean ± σ):      26.0 ms ±   4.9 ms    [User: 8.0 ms, System: 9.4 ms]
  Range (min … max):    18.1 ms …  37.0 ms    86 runs