allow typeCheck CLI flag to override transpileOnly tsconfig flag

[cspotcode]

Fixes #1119

In #1119 we discussed some more ideal solutions, but this fix is simple and unblocks the user.

[codecov]

Codecov Report

Merging #1142 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1142   +/-   ##
=======================================
  Coverage   79.60%   79.60%           
=======================================
  Files           7        7           
  Lines         711      711           
  Branches      158      158           
=======================================
  Hits          566      566           
  Misses         89       89           
  Partials       56       56           

Flag	Coverage Δ
node_10	76.28% <0.00%> (ø)
node_12_15	76.63% <0.00%> (ø)
node_12_16	76.63% <0.00%> (ø)
node_13	79.32% <0.00%> (ø)
node_14	79.32% <0.00%> (ø)
node_14_13_0	78.19% <0.00%> (ø)
node_15	79.32% <0.00%> (ø)
typescript_2_7	79.18% <0.00%> (ø)
typescript_latest	78.48% <0.00%> (ø)
typescript_next	78.34% <0.00%> (ø)
ubuntu	79.32% <0.00%> (ø)
windows	79.46% <0.00%> (ø)

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[coveralls]

Coverage increased (+0.9%) to 87.097% when pulling 6a707b8 on ab/fix-1119 into c1e6e3e on master.