feat: support sandbox instances of salesforce

[zacharygolba]

Closes #3

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Adds sandbox support by adding the ability to pass a --sandbox (or -s, for short) flag to the glide init command. When --sandbox is passed to the init command, a boolean "sandbox": true property is added to the resulting json file. This option instructs the authentication layer of glide to use the correct login url (https://test.salesforce.com).

Users of glide will be able to generate an additional json config for their sandbox instance by running the init command with the additional [path] argument to specify the file name that best suits their projects naming conventions.

glide init https://my-sandbox-instance.salesforce.com glide.sandbox.json --sandbox

To serve data from their sandbox instance rather than using the default production glide.json file, they can simply provide the filename as a positional argument to the serve command.

glide serve glide.sandbox.json

[codecov-io]

Codecov Report

Merging #6 into master will decrease coverage by 0.64%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage      50%   49.35%   -0.65%     
==========================================
  Files          11       11              
  Lines         304      308       +4     
  Branches       44       46       +2     
==========================================
  Hits          152      152              
- Misses        152      156       +4

Impacted Files	Coverage Δ
src/lib.ts	0% <ø> (ø)	:arrow_up:
src/oauth.ts	0% <0%> (ø)	:arrow_up:
src/main.ts	0% <0%> (ø)	:arrow_up:

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[jsalva]

is this going to get merged?