fix: Prevent numbers in scientific notation from being passed to ffmpeg

[srubin]

Calling seekInput, duration, or seekOutput with a number that is represented in js in scientific notation leads to a ffmpeg error:

import ffmpegPath from 'ffmpeg-static';
import ffmpeg from 'fluent-ffmpeg';

ffmpeg.setFfmpegPath(ffmpegPath);

ffmpeg('in.wav')
  .output('out.wav')
  .duration(1e-7)
  .on('error', function(err, stdout, stderr) {
    console.log('An error happened: ' + err.message);
    console.log('ffmpeg standard output:\n' + stdout);
    console.log('ffmpeg standard error:\n' + stderr);
  })
  .run()

results in:

An error happened: ffmpeg exited with code 1: Invalid duration specification for t: 1e-7

ffmpeg standard output:

ffmpeg standard error:
ffmpeg version 4.3.2-tessus  https://evermeet.cx/ffmpeg/  Copyright (c) 2000-2021 the FFmpeg developers
  built with Apple clang version 11.0.0 (clang-1100.0.33.17)
  configuration: --cc=/usr/bin/clang --prefix=/opt/ffmpeg --extra-version=tessus --enable-avisynth --enable-fontconfig --enable-gpl --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvmaf --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-version3 --pkg-config-flags=--static --disable-ffplay
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from 'in.wav':
  Metadata:
    originator_reference: aayHNh!eFSVk
    date            : 2021-02-24
    creation_time   : 15:32:33
    time_reference  : 0
    umid            : 0x060A2B340101010501010F1013000000FB1F13F1E785800019E086DAD77A81D0
  Duration: 00:00:03.73, bitrate: 716 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Invalid duration specification for t: 1e-7

To prevent this, this PR formats numbers in decimal notation before adding them as ffmpeg arguments.