Fail if input file name does not start with a capital letter, end with .3d

[tahina-pro]

This PR resolves #68 , by clarifying the behavior of EverParse for the following example (expected to fail):

$ cat test1.3d
entrypoint typedef struct _point {
  UINT16 x;
  UINT16 y;
} point;
$ everparse.sh test1.3d

Previously, this failed with an obscure error message coming from the underlying call to F*. Now, with this PR, the example fails with a clearer error message:

Fatal error: exception (Failure "Input file name test1.3d must start with a capital letter")

Similarly, I also check that an input file name ends with .3d

I also made these points more prominent in the online documentation for command-line options: a highlighted note mandates input file names to start with a capital letter and end with .3d